<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with Wdesk from Workiva-->
<!--Copyright 2021 Workiva-->
<!--r:95eaad3e-055a-485b-afca-b2bf1c93e6ac,g:ed96853a-9dd8-44c3-8a9d-1d61f9264926-->
<link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member"/>
  <link:roleRef roleURI="http://pubmatic.com/role/Cover" xlink:type="simple" xlink:href="pubm-20210331.xsd#Cover"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/Cover" xlink:type="extended" id="i2b60b186225a42478cd9f84527ab5965_Cover">
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentInformationLineItems_4541e0bb-dc68-48fc-8827-7cac96ce9438" xlink:href="https://xbrl.sec.gov/dei/2020/dei-2020-01-31.xsd#dei_DocumentInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentType_c773de05-6b68-436e-9c63-3ce28b096a8f" xlink:href="https://xbrl.sec.gov/dei/2020/dei-2020-01-31.xsd#dei_DocumentType"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4541e0bb-dc68-48fc-8827-7cac96ce9438" xlink:to="loc_dei_DocumentType_c773de05-6b68-436e-9c63-3ce28b096a8f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentQuarterlyReport_c30ba640-3fc2-4055-9af8-c9bd34b54e5e" xlink:href="https://xbrl.sec.gov/dei/2020/dei-2020-01-31.xsd#dei_DocumentQuarterlyReport"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4541e0bb-dc68-48fc-8827-7cac96ce9438" xlink:to="loc_dei_DocumentQuarterlyReport_c30ba640-3fc2-4055-9af8-c9bd34b54e5e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentTransitionReport_481524c8-bc03-4bf8-b428-a4beeec7f8b8" xlink:href="https://xbrl.sec.gov/dei/2020/dei-2020-01-31.xsd#dei_DocumentTransitionReport"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4541e0bb-dc68-48fc-8827-7cac96ce9438" xlink:to="loc_dei_DocumentTransitionReport_481524c8-bc03-4bf8-b428-a4beeec7f8b8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFileNumber_6e062e3a-5d2c-42cd-809e-7788d3ddd8bf" xlink:href="https://xbrl.sec.gov/dei/2020/dei-2020-01-31.xsd#dei_EntityFileNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4541e0bb-dc68-48fc-8827-7cac96ce9438" xlink:to="loc_dei_EntityFileNumber_6e062e3a-5d2c-42cd-809e-7788d3ddd8bf" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityRegistrantName_521ea807-f298-4997-8aba-0fba49895a7a" xlink:href="https://xbrl.sec.gov/dei/2020/dei-2020-01-31.xsd#dei_EntityRegistrantName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4541e0bb-dc68-48fc-8827-7cac96ce9438" xlink:to="loc_dei_EntityRegistrantName_521ea807-f298-4997-8aba-0fba49895a7a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityIncorporationStateCountryCode_89594e2b-6797-41c5-a90f-8b9818372e09" xlink:href="https://xbrl.sec.gov/dei/2020/dei-2020-01-31.xsd#dei_EntityIncorporationStateCountryCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4541e0bb-dc68-48fc-8827-7cac96ce9438" xlink:to="loc_dei_EntityIncorporationStateCountryCode_89594e2b-6797-41c5-a90f-8b9818372e09" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityTaxIdentificationNumber_7d174a2b-88ad-4757-96f7-f08995f74ebb" xlink:href="https://xbrl.sec.gov/dei/2020/dei-2020-01-31.xsd#dei_EntityTaxIdentificationNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4541e0bb-dc68-48fc-8827-7cac96ce9438" xlink:to="loc_dei_EntityTaxIdentificationNumber_7d174a2b-88ad-4757-96f7-f08995f74ebb" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressAddressLine1_d56c4376-8829-47e2-b160-cb22b52ba6b1" xlink:href="https://xbrl.sec.gov/dei/2020/dei-2020-01-31.xsd#dei_EntityAddressAddressLine1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4541e0bb-dc68-48fc-8827-7cac96ce9438" xlink:to="loc_dei_EntityAddressAddressLine1_d56c4376-8829-47e2-b160-cb22b52ba6b1" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressAddressLine2_b813cce6-5e0c-44b0-a1fe-5fe0f055fb55" xlink:href="https://xbrl.sec.gov/dei/2020/dei-2020-01-31.xsd#dei_EntityAddressAddressLine2"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4541e0bb-dc68-48fc-8827-7cac96ce9438" xlink:to="loc_dei_EntityAddressAddressLine2_b813cce6-5e0c-44b0-a1fe-5fe0f055fb55" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressCityOrTown_eba643ed-b46f-494b-b272-e4081f200581" xlink:href="https://xbrl.sec.gov/dei/2020/dei-2020-01-31.xsd#dei_EntityAddressCityOrTown"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4541e0bb-dc68-48fc-8827-7cac96ce9438" xlink:to="loc_dei_EntityAddressCityOrTown_eba643ed-b46f-494b-b272-e4081f200581" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressStateOrProvince_3180f105-c160-4705-b95a-5c852b8f6a98" xlink:href="https://xbrl.sec.gov/dei/2020/dei-2020-01-31.xsd#dei_EntityAddressStateOrProvince"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4541e0bb-dc68-48fc-8827-7cac96ce9438" xlink:to="loc_dei_EntityAddressStateOrProvince_3180f105-c160-4705-b95a-5c852b8f6a98" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressPostalZipCode_092861e0-51f2-46a7-ac1e-26811b7b0631" xlink:href="https://xbrl.sec.gov/dei/2020/dei-2020-01-31.xsd#dei_EntityAddressPostalZipCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4541e0bb-dc68-48fc-8827-7cac96ce9438" xlink:to="loc_dei_EntityAddressPostalZipCode_092861e0-51f2-46a7-ac1e-26811b7b0631" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_Security12bTitle_fbf903e9-9bf6-4c3f-a9f9-b95cbc268a81" xlink:href="https://xbrl.sec.gov/dei/2020/dei-2020-01-31.xsd#dei_Security12bTitle"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4541e0bb-dc68-48fc-8827-7cac96ce9438" xlink:to="loc_dei_Security12bTitle_fbf903e9-9bf6-4c3f-a9f9-b95cbc268a81" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_TradingSymbol_e12cecd6-8fc6-4411-b782-d0d32d0cd2d1" xlink:href="https://xbrl.sec.gov/dei/2020/dei-2020-01-31.xsd#dei_TradingSymbol"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4541e0bb-dc68-48fc-8827-7cac96ce9438" xlink:to="loc_dei_TradingSymbol_e12cecd6-8fc6-4411-b782-d0d32d0cd2d1" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_SecurityExchangeName_cf814982-7a6f-4069-8d66-545b1e04c2c4" xlink:href="https://xbrl.sec.gov/dei/2020/dei-2020-01-31.xsd#dei_SecurityExchangeName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4541e0bb-dc68-48fc-8827-7cac96ce9438" xlink:to="loc_dei_SecurityExchangeName_cf814982-7a6f-4069-8d66-545b1e04c2c4" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCurrentReportingStatus_022eb73a-a9ed-45d7-b9b1-c1e245288b72" xlink:href="https://xbrl.sec.gov/dei/2020/dei-2020-01-31.xsd#dei_EntityCurrentReportingStatus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4541e0bb-dc68-48fc-8827-7cac96ce9438" xlink:to="loc_dei_EntityCurrentReportingStatus_022eb73a-a9ed-45d7-b9b1-c1e245288b72" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityInteractiveDataCurrent_cb65cb96-59fb-4ff8-833f-912ff78582f9" xlink:href="https://xbrl.sec.gov/dei/2020/dei-2020-01-31.xsd#dei_EntityInteractiveDataCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4541e0bb-dc68-48fc-8827-7cac96ce9438" xlink:to="loc_dei_EntityInteractiveDataCurrent_cb65cb96-59fb-4ff8-833f-912ff78582f9" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFilerCategory_fa1a41ee-6a0a-4488-85d0-977a5ae877b6" xlink:href="https://xbrl.sec.gov/dei/2020/dei-2020-01-31.xsd#dei_EntityFilerCategory"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4541e0bb-dc68-48fc-8827-7cac96ce9438" xlink:to="loc_dei_EntityFilerCategory_fa1a41ee-6a0a-4488-85d0-977a5ae877b6" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntitySmallBusiness_828ad7df-10b1-40d1-959c-665d80f823ae" xlink:href="https://xbrl.sec.gov/dei/2020/dei-2020-01-31.xsd#dei_EntitySmallBusiness"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4541e0bb-dc68-48fc-8827-7cac96ce9438" xlink:to="loc_dei_EntitySmallBusiness_828ad7df-10b1-40d1-959c-665d80f823ae" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityEmergingGrowthCompany_8b3dbe03-1fb1-4a39-bff9-4aec7caf8ec5" xlink:href="https://xbrl.sec.gov/dei/2020/dei-2020-01-31.xsd#dei_EntityEmergingGrowthCompany"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4541e0bb-dc68-48fc-8827-7cac96ce9438" xlink:to="loc_dei_EntityEmergingGrowthCompany_8b3dbe03-1fb1-4a39-bff9-4aec7caf8ec5" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityExTransitionPeriod_1e10677a-3bff-45d0-9350-a1dca6e409c4" xlink:href="https://xbrl.sec.gov/dei/2020/dei-2020-01-31.xsd#dei_EntityExTransitionPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4541e0bb-dc68-48fc-8827-7cac96ce9438" xlink:to="loc_dei_EntityExTransitionPeriod_1e10677a-3bff-45d0-9350-a1dca6e409c4" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityShellCompany_3ee38f0d-bb0c-4a76-b42b-e14534586669" xlink:href="https://xbrl.sec.gov/dei/2020/dei-2020-01-31.xsd#dei_EntityShellCompany"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4541e0bb-dc68-48fc-8827-7cac96ce9438" xlink:to="loc_dei_EntityShellCompany_3ee38f0d-bb0c-4a76-b42b-e14534586669" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCommonStockSharesOutstanding_39eabe7c-d831-4183-84b2-58f2df83ea56" xlink:href="https://xbrl.sec.gov/dei/2020/dei-2020-01-31.xsd#dei_EntityCommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4541e0bb-dc68-48fc-8827-7cac96ce9438" xlink:to="loc_dei_EntityCommonStockSharesOutstanding_39eabe7c-d831-4183-84b2-58f2df83ea56" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_AmendmentFlag_ad4710c7-14db-4816-84bb-3133c06441b0" xlink:href="https://xbrl.sec.gov/dei/2020/dei-2020-01-31.xsd#dei_AmendmentFlag"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4541e0bb-dc68-48fc-8827-7cac96ce9438" xlink:to="loc_dei_AmendmentFlag_ad4710c7-14db-4816-84bb-3133c06441b0" xlink:type="arc" order="23"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCentralIndexKey_0257b859-a8e5-45a7-bc8f-4042b8a5b6e6" xlink:href="https://xbrl.sec.gov/dei/2020/dei-2020-01-31.xsd#dei_EntityCentralIndexKey"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4541e0bb-dc68-48fc-8827-7cac96ce9438" xlink:to="loc_dei_EntityCentralIndexKey_0257b859-a8e5-45a7-bc8f-4042b8a5b6e6" xlink:type="arc" order="24"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_CurrentFiscalYearEndDate_0f87911a-e859-4768-88b8-c986aaf601e9" xlink:href="https://xbrl.sec.gov/dei/2020/dei-2020-01-31.xsd#dei_CurrentFiscalYearEndDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4541e0bb-dc68-48fc-8827-7cac96ce9438" xlink:to="loc_dei_CurrentFiscalYearEndDate_0f87911a-e859-4768-88b8-c986aaf601e9" xlink:type="arc" order="25"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentFiscalYearFocus_2e37f017-1c15-4956-ad6b-a84f7df1c758" xlink:href="https://xbrl.sec.gov/dei/2020/dei-2020-01-31.xsd#dei_DocumentFiscalYearFocus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4541e0bb-dc68-48fc-8827-7cac96ce9438" xlink:to="loc_dei_DocumentFiscalYearFocus_2e37f017-1c15-4956-ad6b-a84f7df1c758" xlink:type="arc" order="26"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentFiscalPeriodFocus_b2c96ce0-4b34-4ef9-acee-43e1ca4c694c" xlink:href="https://xbrl.sec.gov/dei/2020/dei-2020-01-31.xsd#dei_DocumentFiscalPeriodFocus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4541e0bb-dc68-48fc-8827-7cac96ce9438" xlink:to="loc_dei_DocumentFiscalPeriodFocus_b2c96ce0-4b34-4ef9-acee-43e1ca4c694c" xlink:type="arc" order="27"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentPeriodEndDate_a387e7a9-7b63-4adf-a01a-6591cb10d33a" xlink:href="https://xbrl.sec.gov/dei/2020/dei-2020-01-31.xsd#dei_DocumentPeriodEndDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_4541e0bb-dc68-48fc-8827-7cac96ce9438" xlink:to="loc_dei_DocumentPeriodEndDate_a387e7a9-7b63-4adf-a01a-6591cb10d33a" xlink:type="arc" order="28"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentInformationTable_0f3afc91-c777-436b-9acd-42cc73be420a" xlink:href="https://xbrl.sec.gov/dei/2020/dei-2020-01-31.xsd#dei_DocumentInformationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_dei_DocumentInformationLineItems_4541e0bb-dc68-48fc-8827-7cac96ce9438" xlink:to="loc_dei_DocumentInformationTable_0f3afc91-c777-436b-9acd-42cc73be420a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_4b2669b7-cf20-4b92-848d-7166bec81787" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dei_DocumentInformationTable_0f3afc91-c777-436b-9acd-42cc73be420a" xlink:to="loc_us-gaap_StatementClassOfStockAxis_4b2669b7-cf20-4b92-848d-7166bec81787" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_4b2669b7-cf20-4b92-848d-7166bec81787_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_4b2669b7-cf20-4b92-848d-7166bec81787" xlink:to="loc_us-gaap_ClassOfStockDomain_4b2669b7-cf20-4b92-848d-7166bec81787_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_e7a76b8a-9fb4-45ca-b833-f93a2f8592cf" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_4b2669b7-cf20-4b92-848d-7166bec81787" xlink:to="loc_us-gaap_ClassOfStockDomain_e7a76b8a-9fb4-45ca-b833-f93a2f8592cf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_cf36a088-90f4-4ee0-a203-2c3614b248e3" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_e7a76b8a-9fb4-45ca-b833-f93a2f8592cf" xlink:to="loc_us-gaap_CommonClassAMember_cf36a088-90f4-4ee0-a203-2c3614b248e3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_175f488f-f676-4490-b751-8ad7a60c8ea0" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_e7a76b8a-9fb4-45ca-b833-f93a2f8592cf" xlink:to="loc_us-gaap_CommonClassBMember_175f488f-f676-4490-b751-8ad7a60c8ea0" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://pubmatic.com/role/CONDENSEDCONSOLIDATEDBALANCESHEETS" xlink:type="simple" xlink:href="pubm-20210331.xsd#CONDENSEDCONSOLIDATEDBALANCESHEETS"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/CONDENSEDCONSOLIDATEDBALANCESHEETS" xlink:type="extended" id="i6b5104f64bcd427e92e8d3d00f6d4995_CONDENSEDCONSOLIDATEDBALANCESHEETS">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_a6b1ecb0-6358-470d-b5fa-3171b4600a42" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsAbstract_442c93e8-0e83-4b34-afc1-139ca3b4e8db" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AssetsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_a6b1ecb0-6358-470d-b5fa-3171b4600a42" xlink:to="loc_us-gaap_AssetsAbstract_442c93e8-0e83-4b34-afc1-139ca3b4e8db" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsCurrentAbstract_73d55f75-7900-4086-b527-4b911b019cd4" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AssetsCurrentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_442c93e8-0e83-4b34-afc1-139ca3b4e8db" xlink:to="loc_us-gaap_AssetsCurrentAbstract_73d55f75-7900-4086-b527-4b911b019cd4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_e032cc03-a87a-47b2-b968-8ec131572944" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_73d55f75-7900-4086-b527-4b911b019cd4" xlink:to="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_e032cc03-a87a-47b2-b968-8ec131572944" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecuritiesCurrent_5516eb6f-568b-4702-ac1f-3014462262f9" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecuritiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_73d55f75-7900-4086-b527-4b911b019cd4" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecuritiesCurrent_5516eb6f-568b-4702-ac1f-3014462262f9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsReceivableNetCurrent_81a71271-2e26-43ac-ba9f-62fdd28c0b56" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AccountsReceivableNetCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_73d55f75-7900-4086-b527-4b911b019cd4" xlink:to="loc_us-gaap_AccountsReceivableNetCurrent_81a71271-2e26-43ac-ba9f-62fdd28c0b56" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrepaidExpenseAndOtherAssetsCurrent_a9d357ca-74ff-47be-9b22-770096a96348" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_PrepaidExpenseAndOtherAssetsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_73d55f75-7900-4086-b527-4b911b019cd4" xlink:to="loc_us-gaap_PrepaidExpenseAndOtherAssetsCurrent_a9d357ca-74ff-47be-9b22-770096a96348" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsCurrent_6353f450-294c-4d5c-8e25-411323c70df8" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AssetsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_73d55f75-7900-4086-b527-4b911b019cd4" xlink:to="loc_us-gaap_AssetsCurrent_6353f450-294c-4d5c-8e25-411323c70df8" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet_2c048456-e6f2-434f-8fb9-ccc9e9ea5aba" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_PropertyPlantAndEquipmentNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_442c93e8-0e83-4b34-afc1-139ca3b4e8db" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet_2c048456-e6f2-434f-8fb9-ccc9e9ea5aba" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_df83d6b9-52d2-4017-94b2-a1e91831a28d" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_442c93e8-0e83-4b34-afc1-139ca3b4e8db" xlink:to="loc_us-gaap_Goodwill_df83d6b9-52d2-4017-94b2-a1e91831a28d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredIncomeTaxAssetsNet_a9b6e7ca-c2fd-4483-ba9a-2ff594e0c3b3" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DeferredIncomeTaxAssetsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_442c93e8-0e83-4b34-afc1-139ca3b4e8db" xlink:to="loc_us-gaap_DeferredIncomeTaxAssetsNet_a9b6e7ca-c2fd-4483-ba9a-2ff594e0c3b3" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherAssetsNoncurrent_6a73dedd-7791-4015-bb1e-66bdac1ec1c6" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_OtherAssetsNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_442c93e8-0e83-4b34-afc1-139ca3b4e8db" xlink:to="loc_us-gaap_OtherAssetsNoncurrent_6a73dedd-7791-4015-bb1e-66bdac1ec1c6" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Assets_1b4ba1ba-911c-4e78-bec0-6a1c3f18b6a4" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_Assets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_442c93e8-0e83-4b34-afc1-139ca3b4e8db" xlink:to="loc_us-gaap_Assets_1b4ba1ba-911c-4e78-bec0-6a1c3f18b6a4" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_ae97f83c-89cf-4de0-96af-d27c0d8fc7e9" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LiabilitiesAndStockholdersEquityAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_a6b1ecb0-6358-470d-b5fa-3171b4600a42" xlink:to="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_ae97f83c-89cf-4de0-96af-d27c0d8fc7e9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesCurrentAbstract_3de0c83c-1ffd-42c5-a691-e842d8d47eee" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LiabilitiesCurrentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_ae97f83c-89cf-4de0-96af-d27c0d8fc7e9" xlink:to="loc_us-gaap_LiabilitiesCurrentAbstract_3de0c83c-1ffd-42c5-a691-e842d8d47eee" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsPayableCurrent_5b0da47d-7a28-4262-99b1-0c6e54f0b0b8" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AccountsPayableCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_3de0c83c-1ffd-42c5-a691-e842d8d47eee" xlink:to="loc_us-gaap_AccountsPayableCurrent_5b0da47d-7a28-4262-99b1-0c6e54f0b0b8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccruedLiabilitiesCurrent_f0d43804-5adb-42bb-a077-7a243e5e752f" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AccruedLiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_3de0c83c-1ffd-42c5-a691-e842d8d47eee" xlink:to="loc_us-gaap_AccruedLiabilitiesCurrent_f0d43804-5adb-42bb-a077-7a243e5e752f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesCurrent_79409bd3-a4a3-43c1-8a58-197ead124be8" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_3de0c83c-1ffd-42c5-a691-e842d8d47eee" xlink:to="loc_us-gaap_LiabilitiesCurrent_79409bd3-a4a3-43c1-8a58-197ead124be8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredIncomeTaxLiabilitiesNet_0c597dc0-71e5-4d80-8a04-bafc2047e096" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DeferredIncomeTaxLiabilitiesNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_ae97f83c-89cf-4de0-96af-d27c0d8fc7e9" xlink:to="loc_us-gaap_DeferredIncomeTaxLiabilitiesNet_0c597dc0-71e5-4d80-8a04-bafc2047e096" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherLiabilitiesNoncurrent_11e19a06-839d-4e61-a6b8-11a3aab581bd" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_OtherLiabilitiesNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_ae97f83c-89cf-4de0-96af-d27c0d8fc7e9" xlink:to="loc_us-gaap_OtherLiabilitiesNoncurrent_11e19a06-839d-4e61-a6b8-11a3aab581bd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Liabilities_275a62f1-202a-4862-871f-93dc2957e078" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_Liabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_ae97f83c-89cf-4de0-96af-d27c0d8fc7e9" xlink:to="loc_us-gaap_Liabilities_275a62f1-202a-4862-871f-93dc2957e078" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommitmentsAndContingencies_28457169-5725-484f-bec5-8bae8ee17387" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommitmentsAndContingencies"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_ae97f83c-89cf-4de0-96af-d27c0d8fc7e9" xlink:to="loc_us-gaap_CommitmentsAndContingencies_28457169-5725-484f-bec5-8bae8ee17387" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityAbstract_cbd4fe79-3b42-4efb-a298-1ba702b24333" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StockholdersEquityAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_ae97f83c-89cf-4de0-96af-d27c0d8fc7e9" xlink:to="loc_us-gaap_StockholdersEquityAbstract_cbd4fe79-3b42-4efb-a298-1ba702b24333" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockValue_eab4955d-cd34-4684-9b33-6f2ae829a074" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_PreferredStockValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_cbd4fe79-3b42-4efb-a298-1ba702b24333" xlink:to="loc_us-gaap_PreferredStockValue_eab4955d-cd34-4684-9b33-6f2ae829a074" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockValue_f4c3b3ce-8c4b-4e68-9d03-2623abc277f0" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommonStockValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_cbd4fe79-3b42-4efb-a298-1ba702b24333" xlink:to="loc_us-gaap_CommonStockValue_f4c3b3ce-8c4b-4e68-9d03-2623abc277f0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockValue_abbcb3ec-af24-431e-a47f-c2bc5771be2c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_TreasuryStockValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_cbd4fe79-3b42-4efb-a298-1ba702b24333" xlink:to="loc_us-gaap_TreasuryStockValue_abbcb3ec-af24-431e-a47f-c2bc5771be2c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapital_6b9723a0-a4c3-419c-a5c7-bffa04fffeab" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AdditionalPaidInCapital"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_cbd4fe79-3b42-4efb-a298-1ba702b24333" xlink:to="loc_us-gaap_AdditionalPaidInCapital_6b9723a0-a4c3-419c-a5c7-bffa04fffeab" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_fbaa2a55-e1c4-4d33-aeb0-ba13977824d3" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_cbd4fe79-3b42-4efb-a298-1ba702b24333" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_fbaa2a55-e1c4-4d33-aeb0-ba13977824d3" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsAccumulatedDeficit_0063b14e-e42b-42c8-b4b6-beeaf8780525" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RetainedEarningsAccumulatedDeficit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_cbd4fe79-3b42-4efb-a298-1ba702b24333" xlink:to="loc_us-gaap_RetainedEarningsAccumulatedDeficit_0063b14e-e42b-42c8-b4b6-beeaf8780525" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_cf8d7dfa-5d11-4f43-9008-c32dd95ff93b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_cbd4fe79-3b42-4efb-a298-1ba702b24333" xlink:to="loc_us-gaap_StockholdersEquity_cf8d7dfa-5d11-4f43-9008-c32dd95ff93b" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAndStockholdersEquity_7ca1a595-0de3-447b-9d5b-dc4097d77785" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LiabilitiesAndStockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_ae97f83c-89cf-4de0-96af-d27c0d8fc7e9" xlink:to="loc_us-gaap_LiabilitiesAndStockholdersEquity_7ca1a595-0de3-447b-9d5b-dc4097d77785" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_8fc79a6f-3ca7-4a0a-92e6-d517dd587b5d" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_a6b1ecb0-6358-470d-b5fa-3171b4600a42" xlink:to="loc_us-gaap_StatementTable_8fc79a6f-3ca7-4a0a-92e6-d517dd587b5d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_ed8df92d-daa6-4564-bd91-801256c9a9b4" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_8fc79a6f-3ca7-4a0a-92e6-d517dd587b5d" xlink:to="loc_us-gaap_StatementClassOfStockAxis_ed8df92d-daa6-4564-bd91-801256c9a9b4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_ed8df92d-daa6-4564-bd91-801256c9a9b4_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_ed8df92d-daa6-4564-bd91-801256c9a9b4" xlink:to="loc_us-gaap_ClassOfStockDomain_ed8df92d-daa6-4564-bd91-801256c9a9b4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_d791a220-61bb-4c72-82d8-84f0c2edd643" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_ed8df92d-daa6-4564-bd91-801256c9a9b4" xlink:to="loc_us-gaap_ClassOfStockDomain_d791a220-61bb-4c72-82d8-84f0c2edd643" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertiblePreferredStockMember_bf6acb95-d03e-4cf0-acfe-502dd47c074d" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ConvertiblePreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_d791a220-61bb-4c72-82d8-84f0c2edd643" xlink:to="loc_us-gaap_ConvertiblePreferredStockMember_bf6acb95-d03e-4cf0-acfe-502dd47c074d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_552e916a-c442-4e2e-9487-75564805645b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_d791a220-61bb-4c72-82d8-84f0c2edd643" xlink:to="loc_us-gaap_CommonStockMember_552e916a-c442-4e2e-9487-75564805645b" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://pubmatic.com/role/CONDENSEDCONSOLIDATEDBALANCESHEETSParenthetical" xlink:type="simple" xlink:href="pubm-20210331.xsd#CONDENSEDCONSOLIDATEDBALANCESHEETSParenthetical"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/CONDENSEDCONSOLIDATEDBALANCESHEETSParenthetical" xlink:type="extended" id="iaa337c4d345d4d6798edba086633eb30_CONDENSEDCONSOLIDATEDBALANCESHEETSParenthetical">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_f50d243a-4a2c-4b68-ac29-d0b49d7986b1" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockParOrStatedValuePerShare_070194cb-2e7c-496b-b117-edb967129521" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_PreferredStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_f50d243a-4a2c-4b68-ac29-d0b49d7986b1" xlink:to="loc_us-gaap_PreferredStockParOrStatedValuePerShare_070194cb-2e7c-496b-b117-edb967129521" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesAuthorized_984fe79a-de5b-4c29-b40e-6f7b1a7e65e1" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_PreferredStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_f50d243a-4a2c-4b68-ac29-d0b49d7986b1" xlink:to="loc_us-gaap_PreferredStockSharesAuthorized_984fe79a-de5b-4c29-b40e-6f7b1a7e65e1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesIssued_4512a005-9fe1-4cf3-9f0a-ab57e0554312" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_PreferredStockSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_f50d243a-4a2c-4b68-ac29-d0b49d7986b1" xlink:to="loc_us-gaap_PreferredStockSharesIssued_4512a005-9fe1-4cf3-9f0a-ab57e0554312" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesOutstanding_89a7936e-a3a0-4877-8f78-20f34ee3de96" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_PreferredStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_f50d243a-4a2c-4b68-ac29-d0b49d7986b1" xlink:to="loc_us-gaap_PreferredStockSharesOutstanding_89a7936e-a3a0-4877-8f78-20f34ee3de96" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_5deb8b01-5efa-4571-a3ce-2acced9aea47" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommonStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_f50d243a-4a2c-4b68-ac29-d0b49d7986b1" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_5deb8b01-5efa-4571-a3ce-2acced9aea47" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesAuthorized_ed858dcc-9574-432c-9c8d-76b8236ede79" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommonStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_f50d243a-4a2c-4b68-ac29-d0b49d7986b1" xlink:to="loc_us-gaap_CommonStockSharesAuthorized_ed858dcc-9574-432c-9c8d-76b8236ede79" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesIssued_e9f0e255-2bc3-4836-b088-8c2b2761983a" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommonStockSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_f50d243a-4a2c-4b68-ac29-d0b49d7986b1" xlink:to="loc_us-gaap_CommonStockSharesIssued_e9f0e255-2bc3-4836-b088-8c2b2761983a" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_248c32e1-69fc-4790-8242-71f179918a3e" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_f50d243a-4a2c-4b68-ac29-d0b49d7986b1" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_248c32e1-69fc-4790-8242-71f179918a3e" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockShares_6b382786-d8a5-42ab-acc5-2da971f9fe02" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_TreasuryStockShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_f50d243a-4a2c-4b68-ac29-d0b49d7986b1" xlink:to="loc_us-gaap_TreasuryStockShares_6b382786-d8a5-42ab-acc5-2da971f9fe02" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_de681201-010f-4177-aa44-ecb4eaea7912" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_f50d243a-4a2c-4b68-ac29-d0b49d7986b1" xlink:to="loc_us-gaap_StatementTable_de681201-010f-4177-aa44-ecb4eaea7912" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_226edc6b-6687-4de0-b7e7-1f1edc72a614" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_de681201-010f-4177-aa44-ecb4eaea7912" xlink:to="loc_us-gaap_StatementClassOfStockAxis_226edc6b-6687-4de0-b7e7-1f1edc72a614" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_226edc6b-6687-4de0-b7e7-1f1edc72a614_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_226edc6b-6687-4de0-b7e7-1f1edc72a614" xlink:to="loc_us-gaap_ClassOfStockDomain_226edc6b-6687-4de0-b7e7-1f1edc72a614_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_04ea392f-082d-467f-a8ab-a99f0f15b1fa" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_226edc6b-6687-4de0-b7e7-1f1edc72a614" xlink:to="loc_us-gaap_ClassOfStockDomain_04ea392f-082d-467f-a8ab-a99f0f15b1fa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_abceec03-87ef-4414-9139-ac0c6f67b4f1" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_04ea392f-082d-467f-a8ab-a99f0f15b1fa" xlink:to="loc_us-gaap_CommonClassAMember_abceec03-87ef-4414-9139-ac0c6f67b4f1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_b974104b-e55d-4d50-8748-37bab05ed460" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_04ea392f-082d-467f-a8ab-a99f0f15b1fa" xlink:to="loc_us-gaap_CommonClassBMember_b974104b-e55d-4d50-8748-37bab05ed460" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://pubmatic.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFOPERATIONS" xlink:type="simple" xlink:href="pubm-20210331.xsd#CONDENSEDCONSOLIDATEDSTATEMENTSOFOPERATIONS"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFOPERATIONS" xlink:type="extended" id="if228a2363a344a2f8ef41a826219d16b_CONDENSEDCONSOLIDATEDSTATEMENTSOFOPERATIONS"/>
  <link:roleRef roleURI="http://pubmatic.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCOMPREHENSIVEINCOME" xlink:type="simple" xlink:href="pubm-20210331.xsd#CONDENSEDCONSOLIDATEDSTATEMENTSOFCOMPREHENSIVEINCOME"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCOMPREHENSIVEINCOME" xlink:type="extended" id="ia33b3d53295e49d5986b7d0817a3478b_CONDENSEDCONSOLIDATEDSTATEMENTSOFCOMPREHENSIVEINCOME"/>
  <link:roleRef roleURI="http://pubmatic.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITY" xlink:type="simple" xlink:href="pubm-20210331.xsd#CONDENSEDCONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITY"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITY" xlink:type="extended" id="ie28d088773ee43eabfc485f8d9457219_CONDENSEDCONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITY">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_1835f7b1-3c0d-4805-bd81-787f4a509036" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_29b23ca9-fab6-4a20-bf43-51d219dd9ff0" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_IncreaseDecreaseInTemporaryEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_1835f7b1-3c0d-4805-bd81-787f4a509036" xlink:to="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_29b23ca9-fab6-4a20-bf43-51d219dd9ff0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquitySharesOutstanding_cb78e45c-425c-49ce-8789-d24f1cc81208" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_TemporaryEquitySharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_29b23ca9-fab6-4a20-bf43-51d219dd9ff0" xlink:to="loc_us-gaap_TemporaryEquitySharesOutstanding_cb78e45c-425c-49ce-8789-d24f1cc81208" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquityCarryingAmountAttributableToParent_9706860f-325a-48ad-9dfd-96c0b33ff26b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_TemporaryEquityCarryingAmountAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_29b23ca9-fab6-4a20-bf43-51d219dd9ff0" xlink:to="loc_us-gaap_TemporaryEquityCarryingAmountAttributableToParent_9706860f-325a-48ad-9dfd-96c0b33ff26b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquitySharesOutstanding_8c96887e-5248-473e-8d5f-41847933cc89" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_TemporaryEquitySharesOutstanding"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquityCarryingAmountAttributableToParent_ed5ea9a0-a93a-4932-81fc-779a7f9dd139" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_TemporaryEquityCarryingAmountAttributableToParent"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_6e53ae6a-ffe1-4bf2-8542-f3a5ea458549" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_1835f7b1-3c0d-4805-bd81-787f4a509036" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_6e53ae6a-ffe1-4bf2-8542-f3a5ea458549" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharesOutstanding_99ce5968-6941-44aa-828a-007741d24e4b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_6e53ae6a-ffe1-4bf2-8542-f3a5ea458549" xlink:to="loc_us-gaap_SharesOutstanding_99ce5968-6941-44aa-828a-007741d24e4b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_81a517be-7f12-44ed-910d-455fee259169" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_6e53ae6a-ffe1-4bf2-8542-f3a5ea458549" xlink:to="loc_us-gaap_StockholdersEquity_81a517be-7f12-44ed-910d-455fee259169" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_4c29a4a4-0a71-4d6b-a7e4-649f055924c5" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_6e53ae6a-ffe1-4bf2-8542-f3a5ea458549" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_4c29a4a4-0a71-4d6b-a7e4-649f055924c5" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_6fcca746-4b50-4e5f-9bcb-2cd2d28b09db" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_6e53ae6a-ffe1-4bf2-8542-f3a5ea458549" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_6fcca746-4b50-4e5f-9bcb-2cd2d28b09db" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_3838cbd8-64fa-464a-adff-77406c8ddccd" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_6e53ae6a-ffe1-4bf2-8542-f3a5ea458549" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_3838cbd8-64fa-464a-adff-77406c8ddccd" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockSharesAcquired_6ed46050-0f7c-45e9-96c6-5da8c3f4e722" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_TreasuryStockSharesAcquired"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_6e53ae6a-ffe1-4bf2-8542-f3a5ea458549" xlink:to="loc_us-gaap_TreasuryStockSharesAcquired_6ed46050-0f7c-45e9-96c6-5da8c3f4e722" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockValueAcquiredCostMethod_189dc16d-69a8-4e97-b804-6d81a821e820" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_TreasuryStockValueAcquiredCostMethod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_6e53ae6a-ffe1-4bf2-8542-f3a5ea458549" xlink:to="loc_us-gaap_TreasuryStockValueAcquiredCostMethod_189dc16d-69a8-4e97-b804-6d81a821e820" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_11ccf0c5-229b-4e59-9667-fd1786b6c996" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_6e53ae6a-ffe1-4bf2-8542-f3a5ea458549" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_11ccf0c5-229b-4e59-9667-fd1786b6c996" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_c1a5abed-75fa-46ea-8e4e-4ea947b75bbe" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_6e53ae6a-ffe1-4bf2-8542-f3a5ea458549" xlink:to="loc_us-gaap_NetIncomeLoss_c1a5abed-75fa-46ea-8e4e-4ea947b75bbe" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharesOutstanding_445d66b7-e011-49a9-8b80-6feed2df9ee4" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SharesOutstanding"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_3fa34b04-d695-4a89-8c49-08858a81033c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StockholdersEquity"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_e63f3041-8f14-4b8c-9291-5bcd4341d263" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_1835f7b1-3c0d-4805-bd81-787f4a509036" xlink:to="loc_us-gaap_StatementTable_e63f3041-8f14-4b8c-9291-5bcd4341d263" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_415bfd2e-f112-4ec3-9e06-a94198b2f1e0" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_e63f3041-8f14-4b8c-9291-5bcd4341d263" xlink:to="loc_us-gaap_StatementClassOfStockAxis_415bfd2e-f112-4ec3-9e06-a94198b2f1e0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_415bfd2e-f112-4ec3-9e06-a94198b2f1e0_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_415bfd2e-f112-4ec3-9e06-a94198b2f1e0" xlink:to="loc_us-gaap_ClassOfStockDomain_415bfd2e-f112-4ec3-9e06-a94198b2f1e0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_7a82d94e-cc33-48e0-8f05-f0a088351621" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_415bfd2e-f112-4ec3-9e06-a94198b2f1e0" xlink:to="loc_us-gaap_ClassOfStockDomain_7a82d94e-cc33-48e0-8f05-f0a088351621" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertiblePreferredStockMember_e4a42039-4263-48ad-b9bb-573f22f22baa" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ConvertiblePreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_7a82d94e-cc33-48e0-8f05-f0a088351621" xlink:to="loc_us-gaap_ConvertiblePreferredStockMember_e4a42039-4263-48ad-b9bb-573f22f22baa" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_pubm_RedeemableCommonStockMember_f008c941-0c27-4fb4-83e8-fb6554feee51" xlink:href="pubm-20210331.xsd#pubm_RedeemableCommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_7a82d94e-cc33-48e0-8f05-f0a088351621" xlink:to="loc_pubm_RedeemableCommonStockMember_f008c941-0c27-4fb4-83e8-fb6554feee51" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_3451e93f-1c57-4e73-9967-581dc7cff54d" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_e63f3041-8f14-4b8c-9291-5bcd4341d263" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_3451e93f-1c57-4e73-9967-581dc7cff54d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_3451e93f-1c57-4e73-9967-581dc7cff54d_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_3451e93f-1c57-4e73-9967-581dc7cff54d" xlink:to="loc_us-gaap_EquityComponentDomain_3451e93f-1c57-4e73-9967-581dc7cff54d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_3081cb40-1216-44c0-8d0c-29b0f1a38672" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_3451e93f-1c57-4e73-9967-581dc7cff54d" xlink:to="loc_us-gaap_EquityComponentDomain_3081cb40-1216-44c0-8d0c-29b0f1a38672" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_c4642edf-5c8a-4f85-be78-1d2bcd58175b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_3081cb40-1216-44c0-8d0c-29b0f1a38672" xlink:to="loc_us-gaap_CommonStockMember_c4642edf-5c8a-4f85-be78-1d2bcd58175b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockMember_0728e28a-66a9-4d76-96b5-eae84323014c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_TreasuryStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_3081cb40-1216-44c0-8d0c-29b0f1a38672" xlink:to="loc_us-gaap_TreasuryStockMember_0728e28a-66a9-4d76-96b5-eae84323014c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_6dcc25d4-0791-4602-b7e0-9e6d2a5b319c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_3081cb40-1216-44c0-8d0c-29b0f1a38672" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_6dcc25d4-0791-4602-b7e0-9e6d2a5b319c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_7aa4a0bc-be5d-4daa-a809-7b1bd4c0aafb" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_3081cb40-1216-44c0-8d0c-29b0f1a38672" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_7aa4a0bc-be5d-4daa-a809-7b1bd4c0aafb" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_167bef6f-cefc-461c-9ff7-f6ba963a8b75" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_3081cb40-1216-44c0-8d0c-29b0f1a38672" xlink:to="loc_us-gaap_RetainedEarningsMember_167bef6f-cefc-461c-9ff7-f6ba963a8b75" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://pubmatic.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCASHFLOWS" xlink:type="simple" xlink:href="pubm-20210331.xsd#CONDENSEDCONSOLIDATEDSTATEMENTSOFCASHFLOWS"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCASHFLOWS" xlink:type="extended" id="i80cfeb32de9e4a3684fcafe22cf18eef_CONDENSEDCONSOLIDATEDSTATEMENTSOFCASHFLOWS"/>
  <link:roleRef roleURI="http://pubmatic.com/role/OrganizationandDescriptionofBusiness" xlink:type="simple" xlink:href="pubm-20210331.xsd#OrganizationandDescriptionofBusiness"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/OrganizationandDescriptionofBusiness" xlink:type="extended" id="ib94720ef42094eeba65ec3c1cd24d29c_OrganizationandDescriptionofBusiness"/>
  <link:roleRef roleURI="http://pubmatic.com/role/BasisofPresentationandSummaryofSignificantAccountingPolicies" xlink:type="simple" xlink:href="pubm-20210331.xsd#BasisofPresentationandSummaryofSignificantAccountingPolicies"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/BasisofPresentationandSummaryofSignificantAccountingPolicies" xlink:type="extended" id="i951a4c5a64eb482bbf39e9c9dd0f9e84_BasisofPresentationandSummaryofSignificantAccountingPolicies"/>
  <link:roleRef roleURI="http://pubmatic.com/role/BasisofPresentationandSummaryofSignificantAccountingPoliciesPolicies" xlink:type="simple" xlink:href="pubm-20210331.xsd#BasisofPresentationandSummaryofSignificantAccountingPoliciesPolicies"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/BasisofPresentationandSummaryofSignificantAccountingPoliciesPolicies" xlink:type="extended" id="ie11461ed09ec43bf8ee1aed800fd6b4c_BasisofPresentationandSummaryofSignificantAccountingPoliciesPolicies"/>
  <link:roleRef roleURI="http://pubmatic.com/role/BasisofPresentationandSummaryofSignificantAccountingPoliciesInitialPublicOfferingDetails" xlink:type="simple" xlink:href="pubm-20210331.xsd#BasisofPresentationandSummaryofSignificantAccountingPoliciesInitialPublicOfferingDetails"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/BasisofPresentationandSummaryofSignificantAccountingPoliciesInitialPublicOfferingDetails" xlink:type="extended" id="ic4b9ac808b0341db8a23d5c85674e125_BasisofPresentationandSummaryofSignificantAccountingPoliciesInitialPublicOfferingDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_b589bde6-b7b2-40b6-9838-835b7fd2e47a" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ScheduleOfEquityMethodInvestmentsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_53e9821b-ec59-413d-8499-a9798ea10447" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_b589bde6-b7b2-40b6-9838-835b7fd2e47a" xlink:to="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_53e9821b-ec59-413d-8499-a9798ea10447" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockPricePerShare_b70aadc6-9208-486a-bb84-5152ec81286d" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SaleOfStockPricePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_b589bde6-b7b2-40b6-9838-835b7fd2e47a" xlink:to="loc_us-gaap_SaleOfStockPricePerShare_b70aadc6-9208-486a-bb84-5152ec81286d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_ca6762c5-154f-40ae-a58a-b45f1f9d08a7" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SaleOfStockConsiderationReceivedOnTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_b589bde6-b7b2-40b6-9838-835b7fd2e47a" xlink:to="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_ca6762c5-154f-40ae-a58a-b45f1f9d08a7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion_993bf5fe-a49d-4aba-af08-f7759d04b62b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_b589bde6-b7b2-40b6-9838-835b7fd2e47a" xlink:to="loc_us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion_993bf5fe-a49d-4aba-af08-f7759d04b62b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredOfferingCosts_ce492866-77fc-4cf5-b448-49001b38b751" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DeferredOfferingCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_b589bde6-b7b2-40b6-9838-835b7fd2e47a" xlink:to="loc_us-gaap_DeferredOfferingCosts_ce492866-77fc-4cf5-b448-49001b38b751" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEquityMethodInvestmentsTable_2c14fb78-cbfa-4a00-8a44-b373a5721956" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ScheduleOfEquityMethodInvestmentsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_b589bde6-b7b2-40b6-9838-835b7fd2e47a" xlink:to="loc_us-gaap_ScheduleOfEquityMethodInvestmentsTable_2c14fb78-cbfa-4a00-8a44-b373a5721956" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_298f3225-d3c4-4bd7-97f0-8ee857d2ee2b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SubsidiarySaleOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsTable_2c14fb78-cbfa-4a00-8a44-b373a5721956" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_298f3225-d3c4-4bd7-97f0-8ee857d2ee2b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_298f3225-d3c4-4bd7-97f0-8ee857d2ee2b_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_298f3225-d3c4-4bd7-97f0-8ee857d2ee2b" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_298f3225-d3c4-4bd7-97f0-8ee857d2ee2b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_2e8006c4-d76b-448b-b856-94ba1283789e" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_298f3225-d3c4-4bd7-97f0-8ee857d2ee2b" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_2e8006c4-d76b-448b-b856-94ba1283789e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IPOMember_b01f104b-fb73-412e-86f0-8d48799aef09" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_IPOMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_2e8006c4-d76b-448b-b856-94ba1283789e" xlink:to="loc_us-gaap_IPOMember_b01f104b-fb73-412e-86f0-8d48799aef09" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_c1a0d8b6-6a3d-494e-ad1d-5d60810f9f0d" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsTable_2c14fb78-cbfa-4a00-8a44-b373a5721956" xlink:to="loc_us-gaap_StatementClassOfStockAxis_c1a0d8b6-6a3d-494e-ad1d-5d60810f9f0d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_c1a0d8b6-6a3d-494e-ad1d-5d60810f9f0d_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_c1a0d8b6-6a3d-494e-ad1d-5d60810f9f0d" xlink:to="loc_us-gaap_ClassOfStockDomain_c1a0d8b6-6a3d-494e-ad1d-5d60810f9f0d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_9ba93ae0-5533-4c23-90fe-f69f92f4038f" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_c1a0d8b6-6a3d-494e-ad1d-5d60810f9f0d" xlink:to="loc_us-gaap_ClassOfStockDomain_9ba93ae0-5533-4c23-90fe-f69f92f4038f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_b10347ae-05a6-45eb-b042-f221aafe5ae8" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_9ba93ae0-5533-4c23-90fe-f69f92f4038f" xlink:to="loc_us-gaap_CommonClassBMember_b10347ae-05a6-45eb-b042-f221aafe5ae8" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://pubmatic.com/role/BasisofPresentationandSummaryofSignificantAccountingPoliciesConcentrationRiskPercentageDetails" xlink:type="simple" xlink:href="pubm-20210331.xsd#BasisofPresentationandSummaryofSignificantAccountingPoliciesConcentrationRiskPercentageDetails"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/BasisofPresentationandSummaryofSignificantAccountingPoliciesConcentrationRiskPercentageDetails" xlink:type="extended" id="i654a73d924b447429f2f295e078eef33_BasisofPresentationandSummaryofSignificantAccountingPoliciesConcentrationRiskPercentageDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskLineItems_b7b5720d-6f94-4a5f-a938-df50683c5c6b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ConcentrationRiskLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskPercentage1_bf71c68b-315e-4b05-92a1-158aa1f4e177" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ConcentrationRiskPercentage1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_b7b5720d-6f94-4a5f-a938-df50683c5c6b" xlink:to="loc_us-gaap_ConcentrationRiskPercentage1_bf71c68b-315e-4b05-92a1-158aa1f4e177" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTable_0c182d1d-c6e7-4b9b-8d9d-8545225c669d" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ConcentrationRiskTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_b7b5720d-6f94-4a5f-a938-df50683c5c6b" xlink:to="loc_us-gaap_ConcentrationRiskTable_0c182d1d-c6e7-4b9b-8d9d-8545225c669d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByTypeAxis_ec59bad2-21fb-4dcd-9297-f96591244ca9" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ConcentrationRiskByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_0c182d1d-c6e7-4b9b-8d9d-8545225c669d" xlink:to="loc_us-gaap_ConcentrationRiskByTypeAxis_ec59bad2-21fb-4dcd-9297-f96591244ca9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_ec59bad2-21fb-4dcd-9297-f96591244ca9_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_ec59bad2-21fb-4dcd-9297-f96591244ca9" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_ec59bad2-21fb-4dcd-9297-f96591244ca9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_33797f90-f8fa-4e35-9550-e75fe180458f" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_ec59bad2-21fb-4dcd-9297-f96591244ca9" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_33797f90-f8fa-4e35-9550-e75fe180458f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerConcentrationRiskMember_37a80a5e-9f23-40c6-8996-288386f4fb4e" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CustomerConcentrationRiskMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskTypeDomain_33797f90-f8fa-4e35-9550-e75fe180458f" xlink:to="loc_us-gaap_CustomerConcentrationRiskMember_37a80a5e-9f23-40c6-8996-288386f4fb4e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_cda56027-57df-4bf5-9f6a-f92a22118a0b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ConcentrationRiskByBenchmarkAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_0c182d1d-c6e7-4b9b-8d9d-8545225c669d" xlink:to="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_cda56027-57df-4bf5-9f6a-f92a22118a0b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_cda56027-57df-4bf5-9f6a-f92a22118a0b_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_cda56027-57df-4bf5-9f6a-f92a22118a0b" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_cda56027-57df-4bf5-9f6a-f92a22118a0b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_ae4ce8a3-8d8b-4a20-9f14-a7658baad816" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_cda56027-57df-4bf5-9f6a-f92a22118a0b" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_ae4ce8a3-8d8b-4a20-9f14-a7658baad816" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsReceivableMember_952bd03e-782d-47c8-866e-bd5e7d9d2c37" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AccountsReceivableMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_ae4ce8a3-8d8b-4a20-9f14-a7658baad816" xlink:to="loc_us-gaap_AccountsReceivableMember_952bd03e-782d-47c8-866e-bd5e7d9d2c37" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MajorCustomersAxis_9c43e30f-b176-4d4e-8c4c-165c55751da1" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_MajorCustomersAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_0c182d1d-c6e7-4b9b-8d9d-8545225c669d" xlink:to="loc_srt_MajorCustomersAxis_9c43e30f-b176-4d4e-8c4c-165c55751da1" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_NameOfMajorCustomerDomain_9c43e30f-b176-4d4e-8c4c-165c55751da1_default" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_NameOfMajorCustomerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_MajorCustomersAxis_9c43e30f-b176-4d4e-8c4c-165c55751da1" xlink:to="loc_srt_NameOfMajorCustomerDomain_9c43e30f-b176-4d4e-8c4c-165c55751da1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_NameOfMajorCustomerDomain_79a825a6-1ffc-4672-be5c-b3cf78d0a4ad" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_NameOfMajorCustomerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_MajorCustomersAxis_9c43e30f-b176-4d4e-8c4c-165c55751da1" xlink:to="loc_srt_NameOfMajorCustomerDomain_79a825a6-1ffc-4672-be5c-b3cf78d0a4ad" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_pubm_OnePublisherMember_e99aef04-8ae0-4080-8285-a8be8c6c5c5a" xlink:href="pubm-20210331.xsd#pubm_OnePublisherMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_79a825a6-1ffc-4672-be5c-b3cf78d0a4ad" xlink:to="loc_pubm_OnePublisherMember_e99aef04-8ae0-4080-8285-a8be8c6c5c5a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_pubm_BuyerOneMember_e829a24c-70fa-470e-a46d-dc9358b8ddc4" xlink:href="pubm-20210331.xsd#pubm_BuyerOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_79a825a6-1ffc-4672-be5c-b3cf78d0a4ad" xlink:to="loc_pubm_BuyerOneMember_e829a24c-70fa-470e-a46d-dc9358b8ddc4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_pubm_BuyerTwoMember_2f56a29f-9066-409b-9b8a-acf93dd43852" xlink:href="pubm-20210331.xsd#pubm_BuyerTwoMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_79a825a6-1ffc-4672-be5c-b3cf78d0a4ad" xlink:to="loc_pubm_BuyerTwoMember_2f56a29f-9066-409b-9b8a-acf93dd43852" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_pubm_BuyerThreeMember_55a4cb81-6830-4f7b-90b7-3a927ad9d7e2" xlink:href="pubm-20210331.xsd#pubm_BuyerThreeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_79a825a6-1ffc-4672-be5c-b3cf78d0a4ad" xlink:to="loc_pubm_BuyerThreeMember_55a4cb81-6830-4f7b-90b7-3a927ad9d7e2" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_pubm_BuyerFourMember_f4b10351-030f-47d1-99ee-f76e4111b6fb" xlink:href="pubm-20210331.xsd#pubm_BuyerFourMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_79a825a6-1ffc-4672-be5c-b3cf78d0a4ad" xlink:to="loc_pubm_BuyerFourMember_f4b10351-030f-47d1-99ee-f76e4111b6fb" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://pubmatic.com/role/FairValueMeasurements" xlink:type="simple" xlink:href="pubm-20210331.xsd#FairValueMeasurements"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/FairValueMeasurements" xlink:type="extended" id="i7bf529e8edeb4fe1b2ff7ca07516e8f3_FairValueMeasurements"/>
  <link:roleRef roleURI="http://pubmatic.com/role/FairValueMeasurementsTables" xlink:type="simple" xlink:href="pubm-20210331.xsd#FairValueMeasurementsTables"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/FairValueMeasurementsTables" xlink:type="extended" id="i51459b7a671d4de7a86dac4bf63f54f1_FairValueMeasurementsTables"/>
  <link:roleRef roleURI="http://pubmatic.com/role/FairValueMeasurementsDetails" xlink:type="simple" xlink:href="pubm-20210331.xsd#FairValueMeasurementsDetails"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/FairValueMeasurementsDetails" xlink:type="extended" id="i8b6400a2eee04e08bb6807f7ddbecfc6_FairValueMeasurementsDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_58622ad7-2868-43a0-81b7-106ba37dc26b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_20b16e92-ee12-43e9-8af2-02f7cdf66fd3" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CashAndCashEquivalentsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_58622ad7-2868-43a0-81b7-106ba37dc26b" xlink:to="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_20b16e92-ee12-43e9-8af2-02f7cdf66fd3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_fc4a83a1-5d9e-4a7b-9cc1-c2e929fd04d5" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_58622ad7-2868-43a0-81b7-106ba37dc26b" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_fc4a83a1-5d9e-4a7b-9cc1-c2e929fd04d5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_20daa134-b5ce-4816-bae2-3618ce13ed0d" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_58622ad7-2868-43a0-81b7-106ba37dc26b" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_20daa134-b5ce-4816-bae2-3618ce13ed0d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_28f423e1-4e51-4959-a5fa-b7de3c3ee55a" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_20daa134-b5ce-4816-bae2-3618ce13ed0d" xlink:to="loc_us-gaap_FinancialInstrumentAxis_28f423e1-4e51-4959-a5fa-b7de3c3ee55a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_28f423e1-4e51-4959-a5fa-b7de3c3ee55a_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_28f423e1-4e51-4959-a5fa-b7de3c3ee55a" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_28f423e1-4e51-4959-a5fa-b7de3c3ee55a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_6835a19c-3f5a-445f-80b0-3aa11b46ce3f" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_28f423e1-4e51-4959-a5fa-b7de3c3ee55a" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_6835a19c-3f5a-445f-80b0-3aa11b46ce3f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialPaperMember_0d55eb6c-6dad-4511-8ca0-aeba0b51396c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommercialPaperMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_6835a19c-3f5a-445f-80b0-3aa11b46ce3f" xlink:to="loc_us-gaap_CommercialPaperMember_0d55eb6c-6dad-4511-8ca0-aeba0b51396c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USTreasuryAndGovernmentMember_a7461cc9-0478-4ea6-aa2f-49d5bcae9157" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_USTreasuryAndGovernmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_6835a19c-3f5a-445f-80b0-3aa11b46ce3f" xlink:to="loc_us-gaap_USTreasuryAndGovernmentMember_a7461cc9-0478-4ea6-aa2f-49d5bcae9157" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_5805205a-3f7e-40d6-ac07-1f3dfc4ad258" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_20daa134-b5ce-4816-bae2-3618ce13ed0d" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_5805205a-3f7e-40d6-ac07-1f3dfc4ad258" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_5805205a-3f7e-40d6-ac07-1f3dfc4ad258_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_5805205a-3f7e-40d6-ac07-1f3dfc4ad258" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_5805205a-3f7e-40d6-ac07-1f3dfc4ad258_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_2ecef69e-1050-40d3-a429-b0ed08efb18e" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_5805205a-3f7e-40d6-ac07-1f3dfc4ad258" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_2ecef69e-1050-40d3-a429-b0ed08efb18e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_8c55d861-5534-4b7e-ad0d-03dd3f6073ba" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_FairValueInputsLevel1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_2ecef69e-1050-40d3-a429-b0ed08efb18e" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_8c55d861-5534-4b7e-ad0d-03dd3f6073ba" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_4dd78601-ddef-4a8b-ad53-c7481817f7c3" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_2ecef69e-1050-40d3-a429-b0ed08efb18e" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_4dd78601-ddef-4a8b-ad53-c7481817f7c3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_f4eea345-1a06-4612-a6dd-aeb604d965fd" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_2ecef69e-1050-40d3-a429-b0ed08efb18e" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_f4eea345-1a06-4612-a6dd-aeb604d965fd" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAxis_da8d4819-6413-4982-b407-91c2448bac32" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CashAndCashEquivalentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_20daa134-b5ce-4816-bae2-3618ce13ed0d" xlink:to="loc_us-gaap_CashAndCashEquivalentsAxis_da8d4819-6413-4982-b407-91c2448bac32" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_da8d4819-6413-4982-b407-91c2448bac32_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CashAndCashEquivalentsAxis_da8d4819-6413-4982-b407-91c2448bac32" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_da8d4819-6413-4982-b407-91c2448bac32_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_7779ddc8-4a58-4d29-b75d-4770e54fba3a" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CashAndCashEquivalentsAxis_da8d4819-6413-4982-b407-91c2448bac32" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_7779ddc8-4a58-4d29-b75d-4770e54fba3a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashEquivalentsMember_00859bb2-f70f-45ff-86d3-bff3eb1dbd5a" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_7779ddc8-4a58-4d29-b75d-4770e54fba3a" xlink:to="loc_us-gaap_CashEquivalentsMember_00859bb2-f70f-45ff-86d3-bff3eb1dbd5a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementFrequencyAxis_cdf67d03-15c3-4cae-b3a3-a4a8cfabe2f2" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_FairValueByMeasurementFrequencyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_20daa134-b5ce-4816-bae2-3618ce13ed0d" xlink:to="loc_us-gaap_FairValueByMeasurementFrequencyAxis_cdf67d03-15c3-4cae-b3a3-a4a8cfabe2f2" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_cdf67d03-15c3-4cae-b3a3-a4a8cfabe2f2_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_cdf67d03-15c3-4cae-b3a3-a4a8cfabe2f2" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_cdf67d03-15c3-4cae-b3a3-a4a8cfabe2f2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_c9ab2ba6-63cb-4c28-b039-c2dac32298f8" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_cdf67d03-15c3-4cae-b3a3-a4a8cfabe2f2" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_c9ab2ba6-63cb-4c28-b039-c2dac32298f8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsRecurringMember_660591f4-f710-4cf2-9df5-52b1c775566f" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_FairValueMeasurementsRecurringMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementFrequencyDomain_c9ab2ba6-63cb-4c28-b039-c2dac32298f8" xlink:to="loc_us-gaap_FairValueMeasurementsRecurringMember_660591f4-f710-4cf2-9df5-52b1c775566f" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://pubmatic.com/role/BalanceSheetComponents" xlink:type="simple" xlink:href="pubm-20210331.xsd#BalanceSheetComponents"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/BalanceSheetComponents" xlink:type="extended" id="if94795158d2841df8dd84748ca464cbc_BalanceSheetComponents"/>
  <link:roleRef roleURI="http://pubmatic.com/role/BalanceSheetComponentsTables" xlink:type="simple" xlink:href="pubm-20210331.xsd#BalanceSheetComponentsTables"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/BalanceSheetComponentsTables" xlink:type="extended" id="i0b364c260d4f4531801ce34bfc305e45_BalanceSheetComponentsTables"/>
  <link:roleRef roleURI="http://pubmatic.com/role/BalanceSheetComponentsSummaryofMarketableSecuritiesDetails" xlink:type="simple" xlink:href="pubm-20210331.xsd#BalanceSheetComponentsSummaryofMarketableSecuritiesDetails"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/BalanceSheetComponentsSummaryofMarketableSecuritiesDetails" xlink:type="extended" id="iabe99a2cdb2a4ac1825da04b9f98d8f0_BalanceSheetComponentsSummaryofMarketableSecuritiesDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_7f63bddb-1be9-40e9-bbfd-e65d3c784fab" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostCurrent_82134e25-f970-4fe7-a361-89ac2710f089" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_7f63bddb-1be9-40e9-bbfd-e65d3c784fab" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostCurrent_82134e25-f970-4fe7-a361-89ac2710f089" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_0166f7a9-ca5b-4114-9f6c-48d235a72078" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_7f63bddb-1be9-40e9-bbfd-e65d3c784fab" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_0166f7a9-ca5b-4114-9f6c-48d235a72078" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_394e0606-df80-4e09-b342-291c35de41e1" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_7f63bddb-1be9-40e9-bbfd-e65d3c784fab" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_394e0606-df80-4e09-b342-291c35de41e1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecuritiesCurrent_826e0132-67a2-427c-a57c-01a6e9b11f6d" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecuritiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_7f63bddb-1be9-40e9-bbfd-e65d3c784fab" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecuritiesCurrent_826e0132-67a2-427c-a57c-01a6e9b11f6d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_3a0b0167-aafe-410f-b381-55d9e155cf3a" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DebtSecuritiesAvailableForSaleTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_7f63bddb-1be9-40e9-bbfd-e65d3c784fab" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_3a0b0167-aafe-410f-b381-55d9e155cf3a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_7dfef54c-1a41-42b0-b020-03568e888d2b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_3a0b0167-aafe-410f-b381-55d9e155cf3a" xlink:to="loc_us-gaap_FinancialInstrumentAxis_7dfef54c-1a41-42b0-b020-03568e888d2b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_7dfef54c-1a41-42b0-b020-03568e888d2b_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_7dfef54c-1a41-42b0-b020-03568e888d2b" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_7dfef54c-1a41-42b0-b020-03568e888d2b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_bb26b002-1a20-4c57-ba07-a51de2634612" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_7dfef54c-1a41-42b0-b020-03568e888d2b" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_bb26b002-1a20-4c57-ba07-a51de2634612" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialPaperMember_f5dc6359-c6c8-4c72-8162-963ba689e536" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommercialPaperMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_bb26b002-1a20-4c57-ba07-a51de2634612" xlink:to="loc_us-gaap_CommercialPaperMember_f5dc6359-c6c8-4c72-8162-963ba689e536" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USTreasuryAndGovernmentMember_507cb932-f40d-44c7-92ac-3697a68f77f7" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_USTreasuryAndGovernmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_bb26b002-1a20-4c57-ba07-a51de2634612" xlink:to="loc_us-gaap_USTreasuryAndGovernmentMember_507cb932-f40d-44c7-92ac-3697a68f77f7" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://pubmatic.com/role/BalanceSheetComponentsScheduleofPropertyEquipmentandSoftwareNetDetails" xlink:type="simple" xlink:href="pubm-20210331.xsd#BalanceSheetComponentsScheduleofPropertyEquipmentandSoftwareNetDetails"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/BalanceSheetComponentsScheduleofPropertyEquipmentandSoftwareNetDetails" xlink:type="extended" id="i3ff0b8226086404fad92cf5325b9936b_BalanceSheetComponentsScheduleofPropertyEquipmentandSoftwareNetDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_3933c2ea-190b-4784-b106-3e1a25bbe2af" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_PropertyPlantAndEquipmentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentGross_676a9efe-cca1-47f8-84c1-49e2b846c983" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_PropertyPlantAndEquipmentGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_3933c2ea-190b-4784-b106-3e1a25bbe2af" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentGross_676a9efe-cca1-47f8-84c1-49e2b846c983" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_62aea18b-4424-409b-a7e7-1fefffa02f93" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_3933c2ea-190b-4784-b106-3e1a25bbe2af" xlink:to="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_62aea18b-4424-409b-a7e7-1fefffa02f93" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet_508af810-324b-4979-ab87-e0b6868ab31e" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_PropertyPlantAndEquipmentNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_3933c2ea-190b-4784-b106-3e1a25bbe2af" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet_508af810-324b-4979-ab87-e0b6868ab31e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_b82b3760-faa3-4b29-b15b-6b1664e81f4d" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_3933c2ea-190b-4784-b106-3e1a25bbe2af" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_b82b3760-faa3-4b29-b15b-6b1664e81f4d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_a721f510-34f6-4ed2-877c-bc5c70f88da3" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_b82b3760-faa3-4b29-b15b-6b1664e81f4d" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_a721f510-34f6-4ed2-877c-bc5c70f88da3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_a721f510-34f6-4ed2-877c-bc5c70f88da3_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_a721f510-34f6-4ed2-877c-bc5c70f88da3" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_a721f510-34f6-4ed2-877c-bc5c70f88da3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_4e743e74-1150-4e2f-8f03-a8a2a16d831c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_a721f510-34f6-4ed2-877c-bc5c70f88da3" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_4e743e74-1150-4e2f-8f03-a8a2a16d831c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SoftwareDevelopmentMember_1ab431e6-2bd9-436c-8ea8-b976322eea2e" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SoftwareDevelopmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_4e743e74-1150-4e2f-8f03-a8a2a16d831c" xlink:to="loc_us-gaap_SoftwareDevelopmentMember_1ab431e6-2bd9-436c-8ea8-b976322eea2e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TechnologyEquipmentMember_ad33217f-219b-4761-ba8b-a25737d88225" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_TechnologyEquipmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_4e743e74-1150-4e2f-8f03-a8a2a16d831c" xlink:to="loc_us-gaap_TechnologyEquipmentMember_ad33217f-219b-4761-ba8b-a25737d88225" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseholdImprovementsMember_fcf446ab-cfa5-4c36-9a74-1ea740504940" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LeaseholdImprovementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_4e743e74-1150-4e2f-8f03-a8a2a16d831c" xlink:to="loc_us-gaap_LeaseholdImprovementsMember_fcf446ab-cfa5-4c36-9a74-1ea740504940" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FurnitureAndFixturesMember_3cbbac9e-1371-4332-b421-ff29e8cb959c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_FurnitureAndFixturesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_4e743e74-1150-4e2f-8f03-a8a2a16d831c" xlink:to="loc_us-gaap_FurnitureAndFixturesMember_3cbbac9e-1371-4332-b421-ff29e8cb959c" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://pubmatic.com/role/BalanceSheetComponentsNarrativeDetails" xlink:type="simple" xlink:href="pubm-20210331.xsd#BalanceSheetComponentsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/BalanceSheetComponentsNarrativeDetails" xlink:type="extended" id="i2855ae20f011412ea68c6fca4954a245_BalanceSheetComponentsNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_c7ce73d3-2ee7-4c6d-8070-f9ce1ae9913d" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_PropertyPlantAndEquipmentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DepreciationDepletionAndAmortization_678ab6df-3f4e-4897-bfa0-55301de97366" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DepreciationDepletionAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_c7ce73d3-2ee7-4c6d-8070-f9ce1ae9913d" xlink:to="loc_us-gaap_DepreciationDepletionAndAmortization_678ab6df-3f4e-4897-bfa0-55301de97366" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CapitalizedComputerSoftwareAdditions_3017e624-b8a0-40f1-8e16-fae57b664db3" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CapitalizedComputerSoftwareAdditions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_c7ce73d3-2ee7-4c6d-8070-f9ce1ae9913d" xlink:to="loc_us-gaap_CapitalizedComputerSoftwareAdditions_3017e624-b8a0-40f1-8e16-fae57b664db3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentForAmortization_458dbfab-adba-4e09-896d-5eb7292fd0f6" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AdjustmentForAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_c7ce73d3-2ee7-4c6d-8070-f9ce1ae9913d" xlink:to="loc_us-gaap_AdjustmentForAmortization_458dbfab-adba-4e09-896d-5eb7292fd0f6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_25e4d0d8-8ba0-4aaf-94fe-29da0cb0594a" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_c7ce73d3-2ee7-4c6d-8070-f9ce1ae9913d" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_25e4d0d8-8ba0-4aaf-94fe-29da0cb0594a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_6bab1847-c0b1-4eee-b250-6871a36ad0f3" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_25e4d0d8-8ba0-4aaf-94fe-29da0cb0594a" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_6bab1847-c0b1-4eee-b250-6871a36ad0f3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_6bab1847-c0b1-4eee-b250-6871a36ad0f3_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_6bab1847-c0b1-4eee-b250-6871a36ad0f3" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_6bab1847-c0b1-4eee-b250-6871a36ad0f3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_69339f31-9709-4117-ab86-09f2cdfa2b76" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_6bab1847-c0b1-4eee-b250-6871a36ad0f3" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_69339f31-9709-4117-ab86-09f2cdfa2b76" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SoftwareDevelopmentMember_f3fa962a-dd0e-4482-b2b5-c2ffa1b7eb2c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SoftwareDevelopmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_69339f31-9709-4117-ab86-09f2cdfa2b76" xlink:to="loc_us-gaap_SoftwareDevelopmentMember_f3fa962a-dd0e-4482-b2b5-c2ffa1b7eb2c" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://pubmatic.com/role/BalanceSheetComponentsScheduleofAccountsPayableDetails" xlink:type="simple" xlink:href="pubm-20210331.xsd#BalanceSheetComponentsScheduleofAccountsPayableDetails"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/BalanceSheetComponentsScheduleofAccountsPayableDetails" xlink:type="extended" id="i68fe7af0f53b4c02ba6974cbe5ed59b5_BalanceSheetComponentsScheduleofAccountsPayableDetails"/>
  <link:roleRef roleURI="http://pubmatic.com/role/BalanceSheetComponentsScheduleofAccruedExpensesDetails" xlink:type="simple" xlink:href="pubm-20210331.xsd#BalanceSheetComponentsScheduleofAccruedExpensesDetails"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/BalanceSheetComponentsScheduleofAccruedExpensesDetails" xlink:type="extended" id="i78793853f8e84a43bbd0022e1ef53039_BalanceSheetComponentsScheduleofAccruedExpensesDetails"/>
  <link:roleRef roleURI="http://pubmatic.com/role/LoanandSecurityAgreement" xlink:type="simple" xlink:href="pubm-20210331.xsd#LoanandSecurityAgreement"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/LoanandSecurityAgreement" xlink:type="extended" id="if1a22726dccf46289b82c20c2fbba9a8_LoanandSecurityAgreement"/>
  <link:roleRef roleURI="http://pubmatic.com/role/LoanandSecurityAgreementDetails" xlink:type="simple" xlink:href="pubm-20210331.xsd#LoanandSecurityAgreementDetails"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/LoanandSecurityAgreementDetails" xlink:type="extended" id="ifc21a885751f4466b9e6ef8438091032_LoanandSecurityAgreementDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityLineItems_9138eb96-6348-4463-87d4-90c0eb5a6fe1" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LineOfCreditFacilityLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_18ad6060-ec8e-4fed-85f0-30f081e1faf9" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_9138eb96-6348-4463-87d4-90c0eb5a6fe1" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_18ad6060-ec8e-4fed-85f0-30f081e1faf9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_pubm_LineOfCreditFacilityBorrowingCapacityPercentageOfEligibleAccountsReceivableNet_97b1887c-a73b-4c62-9134-1a390468de47" xlink:href="pubm-20210331.xsd#pubm_LineOfCreditFacilityBorrowingCapacityPercentageOfEligibleAccountsReceivableNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_9138eb96-6348-4463-87d4-90c0eb5a6fe1" xlink:to="loc_pubm_LineOfCreditFacilityBorrowingCapacityPercentageOfEligibleAccountsReceivableNet_97b1887c-a73b-4c62-9134-1a390468de47" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage_8c2088cf-b929-4984-8e0a-d3ac20e4f4e9" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_9138eb96-6348-4463-87d4-90c0eb5a6fe1" xlink:to="loc_us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage_8c2088cf-b929-4984-8e0a-d3ac20e4f4e9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_pubm_LineOfCreditFacilityBorrowingCapacityUnusedCapacityFeeClosingBalanceThreshold_48d9aee3-22ec-433a-987c-870817d668c3" xlink:href="pubm-20210331.xsd#pubm_LineOfCreditFacilityBorrowingCapacityUnusedCapacityFeeClosingBalanceThreshold"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_9138eb96-6348-4463-87d4-90c0eb5a6fe1" xlink:to="loc_pubm_LineOfCreditFacilityBorrowingCapacityUnusedCapacityFeeClosingBalanceThreshold_48d9aee3-22ec-433a-987c-870817d668c3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_7c2025b8-3a0e-4b44-9fce-2050b852ae4c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_9138eb96-6348-4463-87d4-90c0eb5a6fe1" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_7c2025b8-3a0e-4b44-9fce-2050b852ae4c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_pubm_DebtInstrumentCovenantQuickRatioMinimum_911e8d37-ae5b-41ca-a1f3-9389e57d8698" xlink:href="pubm-20210331.xsd#pubm_DebtInstrumentCovenantQuickRatioMinimum"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_9138eb96-6348-4463-87d4-90c0eb5a6fe1" xlink:to="loc_pubm_DebtInstrumentCovenantQuickRatioMinimum_911e8d37-ae5b-41ca-a1f3-9389e57d8698" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityTable_70ab4c00-17f2-4c79-88f5-b3ed21f81570" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LineOfCreditFacilityTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_9138eb96-6348-4463-87d4-90c0eb5a6fe1" xlink:to="loc_us-gaap_LineOfCreditFacilityTable_70ab4c00-17f2-4c79-88f5-b3ed21f81570" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_17c2fd6a-4d0d-4c59-a8e3-73b19de64cd6" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_70ab4c00-17f2-4c79-88f5-b3ed21f81570" xlink:to="loc_us-gaap_CreditFacilityAxis_17c2fd6a-4d0d-4c59-a8e3-73b19de64cd6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_17c2fd6a-4d0d-4c59-a8e3-73b19de64cd6_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_17c2fd6a-4d0d-4c59-a8e3-73b19de64cd6" xlink:to="loc_us-gaap_CreditFacilityDomain_17c2fd6a-4d0d-4c59-a8e3-73b19de64cd6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_714ba0c9-eb84-47c1-88eb-26851876d292" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_17c2fd6a-4d0d-4c59-a8e3-73b19de64cd6" xlink:to="loc_us-gaap_CreditFacilityDomain_714ba0c9-eb84-47c1-88eb-26851876d292" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_37a313ec-d7fd-4119-86aa-654091b98341" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_714ba0c9-eb84-47c1-88eb-26851876d292" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_37a313ec-d7fd-4119-86aa-654091b98341" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityAxis_cd86cc02-7c49-4152-9e2d-45b4d6d7da40" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LineOfCreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_70ab4c00-17f2-4c79-88f5-b3ed21f81570" xlink:to="loc_us-gaap_LineOfCreditFacilityAxis_cd86cc02-7c49-4152-9e2d-45b4d6d7da40" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityLenderDomain_cd86cc02-7c49-4152-9e2d-45b4d6d7da40_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LineOfCreditFacilityLenderDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LineOfCreditFacilityAxis_cd86cc02-7c49-4152-9e2d-45b4d6d7da40" xlink:to="loc_us-gaap_LineOfCreditFacilityLenderDomain_cd86cc02-7c49-4152-9e2d-45b4d6d7da40_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityLenderDomain_0ab84d0d-256f-407f-8583-13ad15ae1981" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LineOfCreditFacilityLenderDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LineOfCreditFacilityAxis_cd86cc02-7c49-4152-9e2d-45b4d6d7da40" xlink:to="loc_us-gaap_LineOfCreditFacilityLenderDomain_0ab84d0d-256f-407f-8583-13ad15ae1981" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_pubm_SiliconValleyBankMember_60abb8c2-72b7-4d85-b1d4-5eb07d6495a4" xlink:href="pubm-20210331.xsd#pubm_SiliconValleyBankMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLenderDomain_0ab84d0d-256f-407f-8583-13ad15ae1981" xlink:to="loc_pubm_SiliconValleyBankMember_60abb8c2-72b7-4d85-b1d4-5eb07d6495a4" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://pubmatic.com/role/CommitmentsandContingencies" xlink:type="simple" xlink:href="pubm-20210331.xsd#CommitmentsandContingencies"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/CommitmentsandContingencies" xlink:type="extended" id="i05e8ac3c53694230970e25f07452c12e_CommitmentsandContingencies"/>
  <link:roleRef roleURI="http://pubmatic.com/role/CommitmentsandContingenciesTables" xlink:type="simple" xlink:href="pubm-20210331.xsd#CommitmentsandContingenciesTables"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/CommitmentsandContingenciesTables" xlink:type="extended" id="i04dcfcf0927f4436a041fb345f65f199_CommitmentsandContingenciesTables"/>
  <link:roleRef roleURI="http://pubmatic.com/role/CommitmentsandContingenciesFutureMinimumCommitmentsDetails" xlink:type="simple" xlink:href="pubm-20210331.xsd#CommitmentsandContingenciesFutureMinimumCommitmentsDetails"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/CommitmentsandContingenciesFutureMinimumCommitmentsDetails" xlink:type="extended" id="iffa8eaa3f907415bb1cb00d78f1bc3d7_CommitmentsandContingenciesFutureMinimumCommitmentsDetails"/>
  <link:roleRef roleURI="http://pubmatic.com/role/CommitmentsandContingenciesNarrativeDetails" xlink:type="simple" xlink:href="pubm-20210331.xsd#CommitmentsandContingenciesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/CommitmentsandContingenciesNarrativeDetails" xlink:type="extended" id="i9aed8a1651d34829ae8a49e23bad5ab6_CommitmentsandContingenciesNarrativeDetails"/>
  <link:roleRef roleURI="http://pubmatic.com/role/ConvertiblePreferredStock" xlink:type="simple" xlink:href="pubm-20210331.xsd#ConvertiblePreferredStock"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/ConvertiblePreferredStock" xlink:type="extended" id="i8fe8dd7773c1457fbb05994440e8e9ea_ConvertiblePreferredStock"/>
  <link:roleRef roleURI="http://pubmatic.com/role/ConvertiblePreferredStockDetails" xlink:type="simple" xlink:href="pubm-20210331.xsd#ConvertiblePreferredStockDetails"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/ConvertiblePreferredStockDetails" xlink:type="extended" id="idbb86683d8d742d1bce03f00277f6f1a_ConvertiblePreferredStockDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredUnitsLineItems_63b7e140-d967-4069-b3b6-600f387ca76f" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_PreferredUnitsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion_80bf4043-95cb-419c-b219-bcc41bb09519" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PreferredUnitsLineItems_63b7e140-d967-4069-b3b6-600f387ca76f" xlink:to="loc_us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion_80bf4043-95cb-419c-b219-bcc41bb09519" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquityCarryingAmountAttributableToParent_9f4331a1-2e0c-4326-b85a-d17f85b8b298" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_TemporaryEquityCarryingAmountAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PreferredUnitsLineItems_63b7e140-d967-4069-b3b6-600f387ca76f" xlink:to="loc_us-gaap_TemporaryEquityCarryingAmountAttributableToParent_9f4331a1-2e0c-4326-b85a-d17f85b8b298" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesAuthorized_6f04d622-3594-45a3-825c-26464e51011a" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_PreferredStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PreferredUnitsLineItems_63b7e140-d967-4069-b3b6-600f387ca76f" xlink:to="loc_us-gaap_PreferredStockSharesAuthorized_6f04d622-3594-45a3-825c-26464e51011a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockParOrStatedValuePerShare_e012e0f6-f46c-47f8-a27c-99cb562ff99b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_PreferredStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PreferredUnitsLineItems_63b7e140-d967-4069-b3b6-600f387ca76f" xlink:to="loc_us-gaap_PreferredStockParOrStatedValuePerShare_e012e0f6-f46c-47f8-a27c-99cb562ff99b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfPreferredUnitsTable_88eef50d-48e3-4309-b010-c098c92fffe0" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ScheduleOfPreferredUnitsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PreferredUnitsLineItems_63b7e140-d967-4069-b3b6-600f387ca76f" xlink:to="loc_us-gaap_ScheduleOfPreferredUnitsTable_88eef50d-48e3-4309-b010-c098c92fffe0" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_6bec8a8f-4390-4fe5-bdba-d441e685beb8" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPreferredUnitsTable_88eef50d-48e3-4309-b010-c098c92fffe0" xlink:to="loc_us-gaap_StatementClassOfStockAxis_6bec8a8f-4390-4fe5-bdba-d441e685beb8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_6bec8a8f-4390-4fe5-bdba-d441e685beb8_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_6bec8a8f-4390-4fe5-bdba-d441e685beb8" xlink:to="loc_us-gaap_ClassOfStockDomain_6bec8a8f-4390-4fe5-bdba-d441e685beb8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_f2738e19-96ea-4f19-8e44-ff432f35c01b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_6bec8a8f-4390-4fe5-bdba-d441e685beb8" xlink:to="loc_us-gaap_ClassOfStockDomain_f2738e19-96ea-4f19-8e44-ff432f35c01b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_6674e8d4-6c4e-4bb7-98cd-fcf39fc0b0f9" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_f2738e19-96ea-4f19-8e44-ff432f35c01b" xlink:to="loc_us-gaap_CommonClassBMember_6674e8d4-6c4e-4bb7-98cd-fcf39fc0b0f9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertiblePreferredStockMember_896e4122-5b82-4160-a6e6-8b1af41de2f9" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ConvertiblePreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_f2738e19-96ea-4f19-8e44-ff432f35c01b" xlink:to="loc_us-gaap_ConvertiblePreferredStockMember_896e4122-5b82-4160-a6e6-8b1af41de2f9" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://pubmatic.com/role/StockholdersNotesReceivableandRedeemableCommonStock" xlink:type="simple" xlink:href="pubm-20210331.xsd#StockholdersNotesReceivableandRedeemableCommonStock"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/StockholdersNotesReceivableandRedeemableCommonStock" xlink:type="extended" id="iccdcd2b600684ea5bc39b3e42baf4caf_StockholdersNotesReceivableandRedeemableCommonStock"/>
  <link:roleRef roleURI="http://pubmatic.com/role/StockholdersNotesReceivableandRedeemableCommonStockDetails" xlink:type="simple" xlink:href="pubm-20210331.xsd#StockholdersNotesReceivableandRedeemableCommonStockDetails"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/StockholdersNotesReceivableandRedeemableCommonStockDetails" xlink:type="extended" id="iff3892ec2c404c60ba5ed37beecf3dff_StockholdersNotesReceivableandRedeemableCommonStockDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockLineItems_25160082-a324-438e-980f-acf971ff8966" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ClassOfStockLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_b550f77d-63fe-40f2-b6d4-5214ad27bedc" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DebtInstrumentFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_25160082-a324-438e-980f-acf971ff8966" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_b550f77d-63fe-40f2-b6d4-5214ad27bedc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_35c826fc-0a07-45f2-ac2c-838dd1ae62b7" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_25160082-a324-438e-980f-acf971ff8966" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_35c826fc-0a07-45f2-ac2c-838dd1ae62b7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_pubm_TemporaryEquityCommonStockSharesIssued_f74bbaa7-7959-47ff-8779-54d40816790f" xlink:href="pubm-20210331.xsd#pubm_TemporaryEquityCommonStockSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_25160082-a324-438e-980f-acf971ff8966" xlink:to="loc_pubm_TemporaryEquityCommonStockSharesIssued_f74bbaa7-7959-47ff-8779-54d40816790f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquityCarryingAmountAttributableToParent_dfbb6248-fc12-491b-80a1-959d3307f73d" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_TemporaryEquityCarryingAmountAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_25160082-a324-438e-980f-acf971ff8966" xlink:to="loc_us-gaap_TemporaryEquityCarryingAmountAttributableToParent_dfbb6248-fc12-491b-80a1-959d3307f73d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_970faff4-1252-4f3a-9cc9-9b9c0023bf56" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ScheduleOfStockByClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_25160082-a324-438e-980f-acf971ff8966" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_970faff4-1252-4f3a-9cc9-9b9c0023bf56" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_fdd0950d-79e2-4134-b931-35c13f9290df" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_970faff4-1252-4f3a-9cc9-9b9c0023bf56" xlink:to="loc_us-gaap_StatementClassOfStockAxis_fdd0950d-79e2-4134-b931-35c13f9290df" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_fdd0950d-79e2-4134-b931-35c13f9290df_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_fdd0950d-79e2-4134-b931-35c13f9290df" xlink:to="loc_us-gaap_ClassOfStockDomain_fdd0950d-79e2-4134-b931-35c13f9290df_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_f4813273-15c4-4868-84df-629f3a84abf6" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_fdd0950d-79e2-4134-b931-35c13f9290df" xlink:to="loc_us-gaap_ClassOfStockDomain_f4813273-15c4-4868-84df-629f3a84abf6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_pubm_RedeemableCommonStockMember_75721e3f-2c6d-4089-baef-c747244ae902" xlink:href="pubm-20210331.xsd#pubm_RedeemableCommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_f4813273-15c4-4868-84df-629f3a84abf6" xlink:to="loc_pubm_RedeemableCommonStockMember_75721e3f-2c6d-4089-baef-c747244ae902" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_49f0ee66-8605-44e3-8fdd-8efc41b43302" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_970faff4-1252-4f3a-9cc9-9b9c0023bf56" xlink:to="loc_us-gaap_DebtInstrumentAxis_49f0ee66-8605-44e3-8fdd-8efc41b43302" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_49f0ee66-8605-44e3-8fdd-8efc41b43302_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_49f0ee66-8605-44e3-8fdd-8efc41b43302" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_49f0ee66-8605-44e3-8fdd-8efc41b43302_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_1dfd0fe2-ebc1-41f5-953d-ad8248fb87f6" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_49f0ee66-8605-44e3-8fdd-8efc41b43302" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_1dfd0fe2-ebc1-41f5-953d-ad8248fb87f6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_pubm_TwoPointFourTwoPercentPromissoryNotesDueAugust2021Member_10a87024-94e6-4f86-8f07-1901df46242f" xlink:href="pubm-20210331.xsd#pubm_TwoPointFourTwoPercentPromissoryNotesDueAugust2021Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_1dfd0fe2-ebc1-41f5-953d-ad8248fb87f6" xlink:to="loc_pubm_TwoPointFourTwoPercentPromissoryNotesDueAugust2021Member_10a87024-94e6-4f86-8f07-1901df46242f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_e6c7ab6c-421f-4b55-928f-7d974308a3e5" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_970faff4-1252-4f3a-9cc9-9b9c0023bf56" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_e6c7ab6c-421f-4b55-928f-7d974308a3e5" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_e6c7ab6c-421f-4b55-928f-7d974308a3e5_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_e6c7ab6c-421f-4b55-928f-7d974308a3e5" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_e6c7ab6c-421f-4b55-928f-7d974308a3e5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_ad4a6a26-c89c-48de-93fd-25c79d152e1e" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_e6c7ab6c-421f-4b55-928f-7d974308a3e5" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_ad4a6a26-c89c-48de-93fd-25c79d152e1e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NotesPayableOtherPayablesMember_0a2e2949-6f59-4ed0-b910-2a8fdec5bc93" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_NotesPayableOtherPayablesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_ad4a6a26-c89c-48de-93fd-25c79d152e1e" xlink:to="loc_us-gaap_NotesPayableOtherPayablesMember_0a2e2949-6f59-4ed0-b910-2a8fdec5bc93" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://pubmatic.com/role/StockholdersEquityandStockOptionPlans" xlink:type="simple" xlink:href="pubm-20210331.xsd#StockholdersEquityandStockOptionPlans"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/StockholdersEquityandStockOptionPlans" xlink:type="extended" id="i5816a4b506bf423dadaecd93b914e619_StockholdersEquityandStockOptionPlans"/>
  <link:roleRef roleURI="http://pubmatic.com/role/StockholdersEquityandStockOptionPlansTables" xlink:type="simple" xlink:href="pubm-20210331.xsd#StockholdersEquityandStockOptionPlansTables"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/StockholdersEquityandStockOptionPlansTables" xlink:type="extended" id="i9b3b9a857d9d4436945ba1573ac21cc7_StockholdersEquityandStockOptionPlansTables"/>
  <link:roleRef roleURI="http://pubmatic.com/role/StockholdersEquityandStockOptionPlansNarrativeDetails" xlink:type="simple" xlink:href="pubm-20210331.xsd#StockholdersEquityandStockOptionPlansNarrativeDetails"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/StockholdersEquityandStockOptionPlansNarrativeDetails" xlink:type="extended" id="ic7dae2938de9457b9825fd9f442b1176_StockholdersEquityandStockOptionPlansNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockLineItems_22d70fa1-c2aa-4610-8642-0811c1ed5745" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ClassOfStockLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesAuthorized_2ef40869-dfb9-4017-95fb-35157c999650" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommonStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_22d70fa1-c2aa-4610-8642-0811c1ed5745" xlink:to="loc_us-gaap_CommonStockSharesAuthorized_2ef40869-dfb9-4017-95fb-35157c999650" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_ebcb6ce8-b96a-4ab0-93d0-499180dfa9e5" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommonStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_22d70fa1-c2aa-4610-8642-0811c1ed5745" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_ebcb6ce8-b96a-4ab0-93d0-499180dfa9e5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_e1474a49-7032-4456-ba6b-c5a851de6b92" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommonStockCapitalSharesReservedForFutureIssuance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_22d70fa1-c2aa-4610-8642-0811c1ed5745" xlink:to="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_e1474a49-7032-4456-ba6b-c5a851de6b92" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_5146f864-e8e9-4d03-87af-63bf5c58c75e" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_22d70fa1-c2aa-4610-8642-0811c1ed5745" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_5146f864-e8e9-4d03-87af-63bf5c58c75e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_2f0f46cc-8ea6-48e9-a801-6f3c43a8b6fd" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_22d70fa1-c2aa-4610-8642-0811c1ed5745" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_2f0f46cc-8ea6-48e9-a801-6f3c43a8b6fd" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_dd944825-3bff-4776-a26f-e9b8337c76ef" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_22d70fa1-c2aa-4610-8642-0811c1ed5745" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_dd944825-3bff-4776-a26f-e9b8337c76ef" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentageOfOutstandingStockMaximum_5aa214e1-e55a-4537-9c1c-b447266ecd7b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentageOfOutstandingStockMaximum"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_22d70fa1-c2aa-4610-8642-0811c1ed5745" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentageOfOutstandingStockMaximum_5aa214e1-e55a-4537-9c1c-b447266ecd7b" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_08a7111e-f86e-4c6f-b9f9-54d02aa24bd9" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_22d70fa1-c2aa-4610-8642-0811c1ed5745" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_08a7111e-f86e-4c6f-b9f9-54d02aa24bd9" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardDiscountFromMarketPricePurchaseDate_5af72540-2d26-4fba-b70d-60baa9d3d21b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardDiscountFromMarketPricePurchaseDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_22d70fa1-c2aa-4610-8642-0811c1ed5745" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardDiscountFromMarketPricePurchaseDate_5af72540-2d26-4fba-b70d-60baa9d3d21b" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_pubm_ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumNumberOfSharesPerEmployeeAmount_8b3d68b2-53af-44b9-a348-cfe641ed1f95" xlink:href="pubm-20210331.xsd#pubm_ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumNumberOfSharesPerEmployeeAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_22d70fa1-c2aa-4610-8642-0811c1ed5745" xlink:to="loc_pubm_ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumNumberOfSharesPerEmployeeAmount_8b3d68b2-53af-44b9-a348-cfe641ed1f95" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumNumberOfSharesPerEmployee_4e956665-71a2-4c59-9835-dac226ca7601" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumNumberOfSharesPerEmployee"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_22d70fa1-c2aa-4610-8642-0811c1ed5745" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumNumberOfSharesPerEmployee_4e956665-71a2-4c59-9835-dac226ca7601" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_pubm_ShareBasedCompensationArrangementByShareBasedPaymentAwardAmountWithheldForEmployeeStockPurchasePlan_ec0994e6-699d-4dc3-8b62-c48f372ec023" xlink:href="pubm-20210331.xsd#pubm_ShareBasedCompensationArrangementByShareBasedPaymentAwardAmountWithheldForEmployeeStockPurchasePlan"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_22d70fa1-c2aa-4610-8642-0811c1ed5745" xlink:to="loc_pubm_ShareBasedCompensationArrangementByShareBasedPaymentAwardAmountWithheldForEmployeeStockPurchasePlan_ec0994e6-699d-4dc3-8b62-c48f372ec023" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_d310a165-fcbc-483e-bfc8-509ac7ab5ea1" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ScheduleOfStockByClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_22d70fa1-c2aa-4610-8642-0811c1ed5745" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_d310a165-fcbc-483e-bfc8-509ac7ab5ea1" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_8cd8d04a-f6f9-4415-83c2-0e0a3069b9cf" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_d310a165-fcbc-483e-bfc8-509ac7ab5ea1" xlink:to="loc_us-gaap_StatementClassOfStockAxis_8cd8d04a-f6f9-4415-83c2-0e0a3069b9cf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_8cd8d04a-f6f9-4415-83c2-0e0a3069b9cf_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_8cd8d04a-f6f9-4415-83c2-0e0a3069b9cf" xlink:to="loc_us-gaap_ClassOfStockDomain_8cd8d04a-f6f9-4415-83c2-0e0a3069b9cf_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_403c289f-c5d9-4a8f-8fe7-6fc9345b427c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_8cd8d04a-f6f9-4415-83c2-0e0a3069b9cf" xlink:to="loc_us-gaap_ClassOfStockDomain_403c289f-c5d9-4a8f-8fe7-6fc9345b427c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_db350682-8951-4945-85fe-197fe07afe17" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_403c289f-c5d9-4a8f-8fe7-6fc9345b427c" xlink:to="loc_us-gaap_CommonClassAMember_db350682-8951-4945-85fe-197fe07afe17" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_5aa31319-fd76-480a-b4b6-f6affee85baa" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_403c289f-c5d9-4a8f-8fe7-6fc9345b427c" xlink:to="loc_us-gaap_CommonClassBMember_5aa31319-fd76-480a-b4b6-f6affee85baa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_6231f1cb-9bb7-43ef-9180-80d3d1264f62" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_d310a165-fcbc-483e-bfc8-509ac7ab5ea1" xlink:to="loc_us-gaap_AwardTypeAxis_6231f1cb-9bb7-43ef-9180-80d3d1264f62" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_6231f1cb-9bb7-43ef-9180-80d3d1264f62_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_6231f1cb-9bb7-43ef-9180-80d3d1264f62" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_6231f1cb-9bb7-43ef-9180-80d3d1264f62_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_7b070351-0ff2-456a-936a-c90253543464" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_6231f1cb-9bb7-43ef-9180-80d3d1264f62" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_7b070351-0ff2-456a-936a-c90253543464" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_1a74bf36-3903-4ddd-a132-10dac688133a" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_7b070351-0ff2-456a-936a-c90253543464" xlink:to="loc_us-gaap_EmployeeStockOptionMember_1a74bf36-3903-4ddd-a132-10dac688133a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_cbe6de5b-396c-4370-9311-ed02f84a5f06" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_7b070351-0ff2-456a-936a-c90253543464" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_cbe6de5b-396c-4370-9311-ed02f84a5f06" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember_3b4ec7fa-61ff-4cec-a8d4-4476b697567d" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EmployeeStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_7b070351-0ff2-456a-936a-c90253543464" xlink:to="loc_us-gaap_EmployeeStockMember_3b4ec7fa-61ff-4cec-a8d4-4476b697567d" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://pubmatic.com/role/StockholdersEquityandStockOptionPlansSummaryofStockOptionActivityandRelatedInformationDetails" xlink:type="simple" xlink:href="pubm-20210331.xsd#StockholdersEquityandStockOptionPlansSummaryofStockOptionActivityandRelatedInformationDetails"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/StockholdersEquityandStockOptionPlansSummaryofStockOptionActivityandRelatedInformationDetails" xlink:type="extended" id="i34089513074549be87745615c425a4b1_StockholdersEquityandStockOptionPlansSummaryofStockOptionActivityandRelatedInformationDetails"/>
  <link:roleRef roleURI="http://pubmatic.com/role/StockholdersEquityandStockOptionPlansSummaryofRestrictedStockUnitsandRelatedInformationDetails" xlink:type="simple" xlink:href="pubm-20210331.xsd#StockholdersEquityandStockOptionPlansSummaryofRestrictedStockUnitsandRelatedInformationDetails"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/StockholdersEquityandStockOptionPlansSummaryofRestrictedStockUnitsandRelatedInformationDetails" xlink:type="extended" id="ic879321aab39403696759c89c6cd8d68_StockholdersEquityandStockOptionPlansSummaryofRestrictedStockUnitsandRelatedInformationDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_4bf6f052-acdc-43ad-aed3-7fc131e67faa" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_d55ea6d2-c0be-4912-b243-2c6b2f460f6c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_4bf6f052-acdc-43ad-aed3-7fc131e67faa" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_d55ea6d2-c0be-4912-b243-2c6b2f460f6c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_b17919d6-dd8a-4d7c-bbce-95553ee7deea" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_d55ea6d2-c0be-4912-b243-2c6b2f460f6c" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_b17919d6-dd8a-4d7c-bbce-95553ee7deea" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_ccb76e85-5ebe-490b-bc68-a5f8eb445364" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_d55ea6d2-c0be-4912-b243-2c6b2f460f6c" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_ccb76e85-5ebe-490b-bc68-a5f8eb445364" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_576fe797-8592-4c43-ad67-0305b5ed3a1b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_19081579-1aaf-4408-8249-ad9beb879898" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_4bf6f052-acdc-43ad-aed3-7fc131e67faa" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_19081579-1aaf-4408-8249-ad9beb879898" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_0502eba8-64f5-405d-b41c-1057fecddc65" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_19081579-1aaf-4408-8249-ad9beb879898" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_0502eba8-64f5-405d-b41c-1057fecddc65" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_65e378a1-7c71-4833-8667-84e54081ab65" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_19081579-1aaf-4408-8249-ad9beb879898" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_65e378a1-7c71-4833-8667-84e54081ab65" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_debbc2d6-09be-4aef-9c3c-9dd60303f5df" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_83f957bb-9593-48bf-ace3-5b7b963979df" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_4bf6f052-acdc-43ad-aed3-7fc131e67faa" xlink:to="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_83f957bb-9593-48bf-ace3-5b7b963979df" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_d3fc6aa4-d34c-48a4-8908-cf236edaeaa2" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_83f957bb-9593-48bf-ace3-5b7b963979df" xlink:to="loc_us-gaap_AwardTypeAxis_d3fc6aa4-d34c-48a4-8908-cf236edaeaa2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_d3fc6aa4-d34c-48a4-8908-cf236edaeaa2_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_d3fc6aa4-d34c-48a4-8908-cf236edaeaa2" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_d3fc6aa4-d34c-48a4-8908-cf236edaeaa2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_53a68fdd-951a-43b2-9412-bd114fc5f6b2" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_d3fc6aa4-d34c-48a4-8908-cf236edaeaa2" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_53a68fdd-951a-43b2-9412-bd114fc5f6b2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_6c7e6ac8-92cc-47ce-a7ef-a14c33b9fd4e" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_53a68fdd-951a-43b2-9412-bd114fc5f6b2" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_6c7e6ac8-92cc-47ce-a7ef-a14c33b9fd4e" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://pubmatic.com/role/StockholdersEquityandStockOptionPlansStockbasedCompensationExpenseDetails" xlink:type="simple" xlink:href="pubm-20210331.xsd#StockholdersEquityandStockOptionPlansStockbasedCompensationExpenseDetails"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/StockholdersEquityandStockOptionPlansStockbasedCompensationExpenseDetails" xlink:type="extended" id="i55b97312cd7749e08221da0641e3328c_StockholdersEquityandStockOptionPlansStockbasedCompensationExpenseDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_5ea6e478-c21c-4a4f-9dfd-6b76160d0683" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_96bfe5a1-2b1b-450c-a2bb-34fd5290d50a" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_5ea6e478-c21c-4a4f-9dfd-6b76160d0683" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_96bfe5a1-2b1b-450c-a2bb-34fd5290d50a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_pubm_ShareBasedPaymentArrangementTaxBenefit_89d77145-19f2-4f6a-b77e-4521c6c953f7" xlink:href="pubm-20210331.xsd#pubm_ShareBasedPaymentArrangementTaxBenefit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_5ea6e478-c21c-4a4f-9dfd-6b76160d0683" xlink:to="loc_pubm_ShareBasedPaymentArrangementTaxBenefit_89d77145-19f2-4f6a-b77e-4521c6c953f7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_pubm_ShareBasedPaymentArrangementExpenseNet_65383b1c-070a-4787-a671-8053e6b8f971" xlink:href="pubm-20210331.xsd#pubm_ShareBasedPaymentArrangementExpenseNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_5ea6e478-c21c-4a4f-9dfd-6b76160d0683" xlink:to="loc_pubm_ShareBasedPaymentArrangementExpenseNet_65383b1c-070a-4787-a671-8053e6b8f971" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_d4fc391e-9a46-4ebc-9d1e-c2badfe2fb32" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_5ea6e478-c21c-4a4f-9dfd-6b76160d0683" xlink:to="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_d4fc391e-9a46-4ebc-9d1e-c2badfe2fb32" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_793a5e74-f1d9-40f2-b40f-aae78776d071" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_IncomeStatementLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_d4fc391e-9a46-4ebc-9d1e-c2badfe2fb32" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_793a5e74-f1d9-40f2-b40f-aae78776d071" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_793a5e74-f1d9-40f2-b40f-aae78776d071_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_793a5e74-f1d9-40f2-b40f-aae78776d071" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_793a5e74-f1d9-40f2-b40f-aae78776d071_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_00a8d1c6-e669-4d7b-80cc-ff7af330fdbb" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_793a5e74-f1d9-40f2-b40f-aae78776d071" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_00a8d1c6-e669-4d7b-80cc-ff7af330fdbb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfSalesMember_3f3bdaad-6670-45f5-9b7d-7f35715d1489" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CostOfSalesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_00a8d1c6-e669-4d7b-80cc-ff7af330fdbb" xlink:to="loc_us-gaap_CostOfSalesMember_3f3bdaad-6670-45f5-9b7d-7f35715d1489" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpenseMember_e0f4d13b-eb5c-442b-a134-3404beb6e197" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ResearchAndDevelopmentExpenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_00a8d1c6-e669-4d7b-80cc-ff7af330fdbb" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpenseMember_e0f4d13b-eb5c-442b-a134-3404beb6e197" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingAndMarketingExpenseMember_3ab71b77-4bf0-42c5-9382-a107383eeaae" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SellingAndMarketingExpenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_00a8d1c6-e669-4d7b-80cc-ff7af330fdbb" xlink:to="loc_us-gaap_SellingAndMarketingExpenseMember_3ab71b77-4bf0-42c5-9382-a107383eeaae" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GeneralAndAdministrativeExpenseMember_1251a63f-b02b-4385-bfa1-570c9bb3ebfa" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_GeneralAndAdministrativeExpenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_00a8d1c6-e669-4d7b-80cc-ff7af330fdbb" xlink:to="loc_us-gaap_GeneralAndAdministrativeExpenseMember_1251a63f-b02b-4385-bfa1-570c9bb3ebfa" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://pubmatic.com/role/NetIncomePerShareAttributabletoCommonStockholders" xlink:type="simple" xlink:href="pubm-20210331.xsd#NetIncomePerShareAttributabletoCommonStockholders"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/NetIncomePerShareAttributabletoCommonStockholders" xlink:type="extended" id="i48661f0eea514b889a9bd94c315d7b6e_NetIncomePerShareAttributabletoCommonStockholders"/>
  <link:roleRef roleURI="http://pubmatic.com/role/NetIncomePerShareAttributabletoCommonStockholdersTables" xlink:type="simple" xlink:href="pubm-20210331.xsd#NetIncomePerShareAttributabletoCommonStockholdersTables"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/NetIncomePerShareAttributabletoCommonStockholdersTables" xlink:type="extended" id="iebdcaad927f24a75bcea01910b823ee5_NetIncomePerShareAttributabletoCommonStockholdersTables"/>
  <link:roleRef roleURI="http://pubmatic.com/role/NetIncomePerShareAttributabletoCommonStockholdersBasicandDilutedNetIncomeperShareDetails" xlink:type="simple" xlink:href="pubm-20210331.xsd#NetIncomePerShareAttributabletoCommonStockholdersBasicandDilutedNetIncomeperShareDetails"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/NetIncomePerShareAttributabletoCommonStockholdersBasicandDilutedNetIncomeperShareDetails" xlink:type="extended" id="i79b6a7d506584f929f7deac8c093bc65_NetIncomePerShareAttributabletoCommonStockholdersBasicandDilutedNetIncomeperShareDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareBasicLineItems_d8648319-540b-4d43-81b6-d490f8dd7447" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EarningsPerShareBasicLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersBasicAbstract_fec30aaa-fbb9-4854-8d06-1bece5cc04f5" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_NetIncomeLossAvailableToCommonStockholdersBasicAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareBasicLineItems_d8648319-540b-4d43-81b6-d490f8dd7447" xlink:to="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersBasicAbstract_fec30aaa-fbb9-4854-8d06-1bece5cc04f5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_55f12c3c-fe45-4bde-9a8b-84250d950b2a" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersBasicAbstract_fec30aaa-fbb9-4854-8d06-1bece5cc04f5" xlink:to="loc_us-gaap_NetIncomeLoss_55f12c3c-fe45-4bde-9a8b-84250d950b2a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UndistributedEarningsLossAllocatedToParticipatingSecuritiesBasic_fe7dadff-c47b-4c66-bff7-11127998c590" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_UndistributedEarningsLossAllocatedToParticipatingSecuritiesBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersBasicAbstract_fec30aaa-fbb9-4854-8d06-1bece5cc04f5" xlink:to="loc_us-gaap_UndistributedEarningsLossAllocatedToParticipatingSecuritiesBasic_fe7dadff-c47b-4c66-bff7-11127998c590" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_pubm_ReallocationOfNetIncomeAttributableToCommonStockholders_e5dffaee-a015-46f3-bd4b-fbc3077d9e2f" xlink:href="pubm-20210331.xsd#pubm_ReallocationOfNetIncomeAttributableToCommonStockholders"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersBasicAbstract_fec30aaa-fbb9-4854-8d06-1bece5cc04f5" xlink:to="loc_pubm_ReallocationOfNetIncomeAttributableToCommonStockholders_e5dffaee-a015-46f3-bd4b-fbc3077d9e2f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic_0d1139d8-f7db-49a4-82b0-5bca64da9bda" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersBasicAbstract_fec30aaa-fbb9-4854-8d06-1bece5cc04f5" xlink:to="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic_0d1139d8-f7db-49a4-82b0-5bca64da9bda" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasicAbstract_a3c50bce-a39e-469e-93b7-9bbcd2d7a451" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasicAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareBasicLineItems_d8648319-540b-4d43-81b6-d490f8dd7447" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasicAbstract_a3c50bce-a39e-469e-93b7-9bbcd2d7a451" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_6a6b608b-f1bc-409b-a21b-d61170e521ca" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasicAbstract_a3c50bce-a39e-469e-93b7-9bbcd2d7a451" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_6a6b608b-f1bc-409b-a21b-d61170e521ca" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareBasic_d3056686-06f0-4e58-bd7a-ba99b5bed7f7" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EarningsPerShareBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareBasicLineItems_d8648319-540b-4d43-81b6-d490f8dd7447" xlink:to="loc_us-gaap_EarningsPerShareBasic_d3056686-06f0-4e58-bd7a-ba99b5bed7f7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersDilutedAbstract_0c7c2849-56c3-43cb-bc0b-e614a260ba42" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_NetIncomeLossAvailableToCommonStockholdersDilutedAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareBasicLineItems_d8648319-540b-4d43-81b6-d490f8dd7447" xlink:to="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersDilutedAbstract_0c7c2849-56c3-43cb-bc0b-e614a260ba42" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersDiluted_546c271e-7950-4c88-a29e-6ea9aa1c75f7" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_NetIncomeLossAvailableToCommonStockholdersDiluted"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersDilutedAbstract_0c7c2849-56c3-43cb-bc0b-e614a260ba42" xlink:to="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersDiluted_546c271e-7950-4c88-a29e-6ea9aa1c75f7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_9765084b-b3ba-4473-905b-6b1258607f01" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareBasicLineItems_d8648319-540b-4d43-81b6-d490f8dd7447" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_9765084b-b3ba-4473-905b-6b1258607f01" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_96532352-5e14-4fcb-8940-19a7fb0f5473" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasic"/>
    <link:loc xlink:type="locator" xlink:label="loc_pubm_IncrementalCommonSharesAttributableToDilutiveEffectOfCallOptions_c33ce2bd-a86b-497b-821a-d2bf9abe5516" xlink:href="pubm-20210331.xsd#pubm_IncrementalCommonSharesAttributableToDilutiveEffectOfCallOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_9765084b-b3ba-4473-905b-6b1258607f01" xlink:to="loc_pubm_IncrementalCommonSharesAttributableToDilutiveEffectOfCallOptions_c33ce2bd-a86b-497b-821a-d2bf9abe5516" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_pubm_IncrementalCommonSharesAttributableToDilutiveEffectOfWarrants_d109000e-d210-42d7-a2f8-60d323ceb5d3" xlink:href="pubm-20210331.xsd#pubm_IncrementalCommonSharesAttributableToDilutiveEffectOfWarrants"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_9765084b-b3ba-4473-905b-6b1258607f01" xlink:to="loc_pubm_IncrementalCommonSharesAttributableToDilutiveEffectOfWarrants_d109000e-d210-42d7-a2f8-60d323ceb5d3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_ecfe2363-56a3-47fe-af9e-bccadb9dd1ab" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_9765084b-b3ba-4473-905b-6b1258607f01" xlink:to="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_ecfe2363-56a3-47fe-af9e-bccadb9dd1ab" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareDiluted_5fea3a56-5532-4ed8-a118-c9565b093094" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EarningsPerShareDiluted"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareBasicLineItems_d8648319-540b-4d43-81b6-d490f8dd7447" xlink:to="loc_us-gaap_EarningsPerShareDiluted_5fea3a56-5532-4ed8-a118-c9565b093094" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEarningsPerShareDilutedByCommonClassTable_02cc507a-b22b-4035-816b-c3992961e3e1" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ScheduleOfEarningsPerShareDilutedByCommonClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EarningsPerShareBasicLineItems_d8648319-540b-4d43-81b6-d490f8dd7447" xlink:to="loc_us-gaap_ScheduleOfEarningsPerShareDilutedByCommonClassTable_02cc507a-b22b-4035-816b-c3992961e3e1" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_17634b95-0a5c-470e-b8a6-5c9ff334c51f" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEarningsPerShareDilutedByCommonClassTable_02cc507a-b22b-4035-816b-c3992961e3e1" xlink:to="loc_us-gaap_StatementClassOfStockAxis_17634b95-0a5c-470e-b8a6-5c9ff334c51f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_17634b95-0a5c-470e-b8a6-5c9ff334c51f_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_17634b95-0a5c-470e-b8a6-5c9ff334c51f" xlink:to="loc_us-gaap_ClassOfStockDomain_17634b95-0a5c-470e-b8a6-5c9ff334c51f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_bc41ba3f-134f-44dc-8862-a2e60239dd10" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_17634b95-0a5c-470e-b8a6-5c9ff334c51f" xlink:to="loc_us-gaap_ClassOfStockDomain_bc41ba3f-134f-44dc-8862-a2e60239dd10" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_c047f6d2-5558-421f-b837-3ee01ed7a266" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_bc41ba3f-134f-44dc-8862-a2e60239dd10" xlink:to="loc_us-gaap_CommonClassAMember_c047f6d2-5558-421f-b837-3ee01ed7a266" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_445e6843-5742-4ce6-984d-408e1194532e" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_bc41ba3f-134f-44dc-8862-a2e60239dd10" xlink:to="loc_us-gaap_CommonClassBMember_445e6843-5742-4ce6-984d-408e1194532e" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://pubmatic.com/role/NetIncomePerShareAttributabletoCommonStockholdersScheduleofAntidilutiveSecuritiesExcludedfromComputationofEarningsperShareDetails" xlink:type="simple" xlink:href="pubm-20210331.xsd#NetIncomePerShareAttributabletoCommonStockholdersScheduleofAntidilutiveSecuritiesExcludedfromComputationofEarningsperShareDetails"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/NetIncomePerShareAttributabletoCommonStockholdersScheduleofAntidilutiveSecuritiesExcludedfromComputationofEarningsperShareDetails" xlink:type="extended" id="i13ff3fc4803c45d5813a3da4b1e01e0e_NetIncomePerShareAttributabletoCommonStockholdersScheduleofAntidilutiveSecuritiesExcludedfromComputationofEarningsperShareDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareDilutedLineItems_33d697a9-d102-46e1-8a2b-0de996a30dd2" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EarningsPerShareDilutedLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_25ee37f7-c303-4a84-94ce-21ae2acc018a" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareDilutedLineItems_33d697a9-d102-46e1-8a2b-0de996a30dd2" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_25ee37f7-c303-4a84-94ce-21ae2acc018a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEarningsPerShareDilutedByCommonClassTable_236786f2-3890-4f7c-aded-db5490c16658" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ScheduleOfEarningsPerShareDilutedByCommonClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EarningsPerShareDilutedLineItems_33d697a9-d102-46e1-8a2b-0de996a30dd2" xlink:to="loc_us-gaap_ScheduleOfEarningsPerShareDilutedByCommonClassTable_236786f2-3890-4f7c-aded-db5490c16658" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_86012b70-25f6-4cbe-adc1-d43e51dc6baa" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEarningsPerShareDilutedByCommonClassTable_236786f2-3890-4f7c-aded-db5490c16658" xlink:to="loc_us-gaap_StatementClassOfStockAxis_86012b70-25f6-4cbe-adc1-d43e51dc6baa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_86012b70-25f6-4cbe-adc1-d43e51dc6baa_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_86012b70-25f6-4cbe-adc1-d43e51dc6baa" xlink:to="loc_us-gaap_ClassOfStockDomain_86012b70-25f6-4cbe-adc1-d43e51dc6baa_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_a3f53e34-2eac-4f22-9111-5b2db4f494d9" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_86012b70-25f6-4cbe-adc1-d43e51dc6baa" xlink:to="loc_us-gaap_ClassOfStockDomain_a3f53e34-2eac-4f22-9111-5b2db4f494d9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_6968ed7b-9b5b-4cfc-a260-a5551e22f829" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_a3f53e34-2eac-4f22-9111-5b2db4f494d9" xlink:to="loc_us-gaap_CommonClassAMember_6968ed7b-9b5b-4cfc-a260-a5551e22f829" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_71d6a61b-646e-4cdc-9567-b41c3ca6bcff" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_a3f53e34-2eac-4f22-9111-5b2db4f494d9" xlink:to="loc_us-gaap_CommonClassBMember_71d6a61b-646e-4cdc-9567-b41c3ca6bcff" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_8a7a42ad-4871-4cce-81f8-414e35cba977" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEarningsPerShareDilutedByCommonClassTable_236786f2-3890-4f7c-aded-db5490c16658" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_8a7a42ad-4871-4cce-81f8-414e35cba977" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_8a7a42ad-4871-4cce-81f8-414e35cba977_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_8a7a42ad-4871-4cce-81f8-414e35cba977" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_8a7a42ad-4871-4cce-81f8-414e35cba977_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_a5e9f04a-e7c8-448c-85c3-82cf53a4f4ef" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_8a7a42ad-4871-4cce-81f8-414e35cba977" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_a5e9f04a-e7c8-448c-85c3-82cf53a4f4ef" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_3b293803-91e3-4cf7-964d-0e647a37851b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_a5e9f04a-e7c8-448c-85c3-82cf53a4f4ef" xlink:to="loc_us-gaap_EmployeeStockOptionMember_3b293803-91e3-4cf7-964d-0e647a37851b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertiblePreferredStockMember_3e244140-a080-4318-889e-f37539d934f8" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ConvertiblePreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_a5e9f04a-e7c8-448c-85c3-82cf53a4f4ef" xlink:to="loc_us-gaap_ConvertiblePreferredStockMember_3e244140-a080-4318-889e-f37539d934f8" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://pubmatic.com/role/IncomeTaxes" xlink:type="simple" xlink:href="pubm-20210331.xsd#IncomeTaxes"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/IncomeTaxes" xlink:type="extended" id="i25d43ebb7eba452499b963d66cb4ca67_IncomeTaxes"/>
  <link:roleRef roleURI="http://pubmatic.com/role/IncomeTaxesNarrativeDetails" xlink:type="simple" xlink:href="pubm-20210331.xsd#IncomeTaxesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/IncomeTaxesNarrativeDetails" xlink:type="extended" id="i718017ef5a9f423db3e28d1c330d1266_IncomeTaxesNarrativeDetails"/>
  <link:roleRef roleURI="http://pubmatic.com/role/SegmentInformation" xlink:type="simple" xlink:href="pubm-20210331.xsd#SegmentInformation"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/SegmentInformation" xlink:type="extended" id="idf5baa4e2b5a4f0483268467a69d043b_SegmentInformation"/>
  <link:roleRef roleURI="http://pubmatic.com/role/SegmentReportingTables" xlink:type="simple" xlink:href="pubm-20210331.xsd#SegmentReportingTables"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/SegmentReportingTables" xlink:type="extended" id="i74d47ce056d34179a6a864e7333ecbd4_SegmentReportingTables"/>
  <link:roleRef roleURI="http://pubmatic.com/role/SegmentReportingDetails" xlink:type="simple" xlink:href="pubm-20210331.xsd#SegmentReportingDetails"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/SegmentReportingDetails" xlink:type="extended" id="i4157af7a95c0414b884cbcb509b03040_SegmentReportingDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_51f2cd25-1986-4e4f-9e24-0f5bbbb7a6cc" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_eec74158-f17c-4b30-ba44-715970a1d4c1" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_51f2cd25-1986-4e4f-9e24-0f5bbbb7a6cc" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_eec74158-f17c-4b30-ba44-715970a1d4c1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoncurrentAssets_6713cdd5-64ed-4b61-866a-cde592537584" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_NoncurrentAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_51f2cd25-1986-4e4f-9e24-0f5bbbb7a6cc" xlink:to="loc_us-gaap_NoncurrentAssets_6713cdd5-64ed-4b61-866a-cde592537584" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable_9c3b40a3-7e3e-4b11-a3e7-a949bde6e722" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_51f2cd25-1986-4e4f-9e24-0f5bbbb7a6cc" xlink:to="loc_us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable_9c3b40a3-7e3e-4b11-a3e7-a949bde6e722" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_3654e0f4-3cfb-493c-91c0-c2bc721ccc94" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable_9c3b40a3-7e3e-4b11-a3e7-a949bde6e722" xlink:to="loc_srt_StatementGeographicalAxis_3654e0f4-3cfb-493c-91c0-c2bc721ccc94" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_3654e0f4-3cfb-493c-91c0-c2bc721ccc94_default" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_3654e0f4-3cfb-493c-91c0-c2bc721ccc94" xlink:to="loc_srt_SegmentGeographicalDomain_3654e0f4-3cfb-493c-91c0-c2bc721ccc94_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_d4bf7071-1a29-4bb7-829b-4a5ffbfded78" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_3654e0f4-3cfb-493c-91c0-c2bc721ccc94" xlink:to="loc_srt_SegmentGeographicalDomain_d4bf7071-1a29-4bb7-829b-4a5ffbfded78" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_bb6ca9a5-65af-44f8-998d-c17afdb88037" xlink:href="https://xbrl.sec.gov/country/2020/country-2020-01-31.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_d4bf7071-1a29-4bb7-829b-4a5ffbfded78" xlink:to="loc_country_US_bb6ca9a5-65af-44f8-998d-c17afdb88037" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EMEAMember_8d8db366-6232-45ed-86d5-88c14b17430b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EMEAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_d4bf7071-1a29-4bb7-829b-4a5ffbfded78" xlink:to="loc_us-gaap_EMEAMember_8d8db366-6232-45ed-86d5-88c14b17430b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_AsiaPacificMember_e4b76512-d0e7-40a3-909d-5d4c6661738c" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_AsiaPacificMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_d4bf7071-1a29-4bb7-829b-4a5ffbfded78" xlink:to="loc_srt_AsiaPacificMember_e4b76512-d0e7-40a3-909d-5d4c6661738c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_pubm_RestOfTheWorldMember_3cea6eaa-0fb7-4710-bcb8-46cff1a375b5" xlink:href="pubm-20210331.xsd#pubm_RestOfTheWorldMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_d4bf7071-1a29-4bb7-829b-4a5ffbfded78" xlink:to="loc_pubm_RestOfTheWorldMember_3cea6eaa-0fb7-4710-bcb8-46cff1a375b5" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://pubmatic.com/role/A401kPlan" xlink:type="simple" xlink:href="pubm-20210331.xsd#A401kPlan"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/A401kPlan" xlink:type="extended" id="i1b3e042c5a5f402f85381b018918314b_A401kPlan"/>
  <link:roleRef roleURI="http://pubmatic.com/role/A401kPlanNarrativeDetails" xlink:type="simple" xlink:href="pubm-20210331.xsd#A401kPlanNarrativeDetails"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/A401kPlanNarrativeDetails" xlink:type="extended" id="ic516b4ab80b142e4a1eaa74094c9dea8_A401kPlanNarrativeDetails"/>
  <link:roleRef roleURI="http://pubmatic.com/role/SubsequentEvents" xlink:type="simple" xlink:href="pubm-20210331.xsd#SubsequentEvents"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/SubsequentEvents" xlink:type="extended" id="i5d4b287e8aeb45a7b4b487d483e88008_SubsequentEvents"/>
</link:linkbase>
