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

<!--XBRL Document Created with Wdesk from Workiva-->
<!--Copyright 2021 Workiva-->
<!--r:75f428cd-810b-488d-9896-455147197020,g:aad16af4-74fb-4864-8fb4-6544e9846fac-->
<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-20210630.xsd#Cover"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/Cover" xlink:type="extended" id="ie55eead561bf4d4bb14bddea4f969e1e_Cover">
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentInformationLineItems_32a0023a-04af-4e9e-a3bc-a0db884a9339" 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_ee5f982a-18c4-48b3-bed9-18044777bd32" 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_32a0023a-04af-4e9e-a3bc-a0db884a9339" xlink:to="loc_dei_DocumentType_ee5f982a-18c4-48b3-bed9-18044777bd32" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentQuarterlyReport_1635843a-42f1-4f93-9fe8-8111d5545d5d" 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_32a0023a-04af-4e9e-a3bc-a0db884a9339" xlink:to="loc_dei_DocumentQuarterlyReport_1635843a-42f1-4f93-9fe8-8111d5545d5d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentPeriodEndDate_4c0b2136-0f59-4665-9c91-907e3017d157" 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_32a0023a-04af-4e9e-a3bc-a0db884a9339" xlink:to="loc_dei_DocumentPeriodEndDate_4c0b2136-0f59-4665-9c91-907e3017d157" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentTransitionReport_bba5c338-d817-4b74-8e60-b869601b5c7e" 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_32a0023a-04af-4e9e-a3bc-a0db884a9339" xlink:to="loc_dei_DocumentTransitionReport_bba5c338-d817-4b74-8e60-b869601b5c7e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFileNumber_2df119a8-c31f-46f2-9521-d0828432eaea" 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_32a0023a-04af-4e9e-a3bc-a0db884a9339" xlink:to="loc_dei_EntityFileNumber_2df119a8-c31f-46f2-9521-d0828432eaea" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityRegistrantName_e589c88f-5a9b-471b-bbe1-5417d549d107" 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_32a0023a-04af-4e9e-a3bc-a0db884a9339" xlink:to="loc_dei_EntityRegistrantName_e589c88f-5a9b-471b-bbe1-5417d549d107" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityIncorporationStateCountryCode_67951d4e-4ff9-49d4-8334-550a1c4db8fb" 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_32a0023a-04af-4e9e-a3bc-a0db884a9339" xlink:to="loc_dei_EntityIncorporationStateCountryCode_67951d4e-4ff9-49d4-8334-550a1c4db8fb" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityTaxIdentificationNumber_cfd0ecb4-439f-4e45-885e-0ceec1d608e4" 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_32a0023a-04af-4e9e-a3bc-a0db884a9339" xlink:to="loc_dei_EntityTaxIdentificationNumber_cfd0ecb4-439f-4e45-885e-0ceec1d608e4" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressAddressLine1_ddde96e7-53b5-478b-91d9-5553fcad34c0" 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_32a0023a-04af-4e9e-a3bc-a0db884a9339" xlink:to="loc_dei_EntityAddressAddressLine1_ddde96e7-53b5-478b-91d9-5553fcad34c0" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressAddressLine2_bcd01ceb-7b7e-4766-9ce2-85199dbf19f7" 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_32a0023a-04af-4e9e-a3bc-a0db884a9339" xlink:to="loc_dei_EntityAddressAddressLine2_bcd01ceb-7b7e-4766-9ce2-85199dbf19f7" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressCityOrTown_83297195-9a7a-418c-8ab4-9f296649cbea" 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_32a0023a-04af-4e9e-a3bc-a0db884a9339" xlink:to="loc_dei_EntityAddressCityOrTown_83297195-9a7a-418c-8ab4-9f296649cbea" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressStateOrProvince_5bb71d20-1ab8-46c5-ad23-65ca85f583e9" 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_32a0023a-04af-4e9e-a3bc-a0db884a9339" xlink:to="loc_dei_EntityAddressStateOrProvince_5bb71d20-1ab8-46c5-ad23-65ca85f583e9" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressPostalZipCode_a1e253e6-3be6-432b-9b0a-41b53bf9a57c" 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_32a0023a-04af-4e9e-a3bc-a0db884a9339" xlink:to="loc_dei_EntityAddressPostalZipCode_a1e253e6-3be6-432b-9b0a-41b53bf9a57c" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_Security12bTitle_44fb987e-470d-4f1f-b8de-85bd6f1049bf" 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_32a0023a-04af-4e9e-a3bc-a0db884a9339" xlink:to="loc_dei_Security12bTitle_44fb987e-470d-4f1f-b8de-85bd6f1049bf" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_TradingSymbol_575e4c3d-74bb-494f-ba7d-efafcf045790" 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_32a0023a-04af-4e9e-a3bc-a0db884a9339" xlink:to="loc_dei_TradingSymbol_575e4c3d-74bb-494f-ba7d-efafcf045790" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_SecurityExchangeName_59515be3-36d8-4997-bcd4-1efde643d5ba" 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_32a0023a-04af-4e9e-a3bc-a0db884a9339" xlink:to="loc_dei_SecurityExchangeName_59515be3-36d8-4997-bcd4-1efde643d5ba" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCurrentReportingStatus_5cc5175c-27e3-4f13-9bd7-fafe9f60ff51" 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_32a0023a-04af-4e9e-a3bc-a0db884a9339" xlink:to="loc_dei_EntityCurrentReportingStatus_5cc5175c-27e3-4f13-9bd7-fafe9f60ff51" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityInteractiveDataCurrent_d5f27ad6-7211-49d1-b3e5-3d1a45a5fe7b" 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_32a0023a-04af-4e9e-a3bc-a0db884a9339" xlink:to="loc_dei_EntityInteractiveDataCurrent_d5f27ad6-7211-49d1-b3e5-3d1a45a5fe7b" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFilerCategory_59e70bc3-9991-416d-b4dd-eaa42450b9dc" 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_32a0023a-04af-4e9e-a3bc-a0db884a9339" xlink:to="loc_dei_EntityFilerCategory_59e70bc3-9991-416d-b4dd-eaa42450b9dc" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntitySmallBusiness_ecd06803-d2e2-486e-bb53-4d686fcd78ac" 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_32a0023a-04af-4e9e-a3bc-a0db884a9339" xlink:to="loc_dei_EntitySmallBusiness_ecd06803-d2e2-486e-bb53-4d686fcd78ac" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityEmergingGrowthCompany_688e4fc6-ca73-4a49-984a-50e83a922026" 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_32a0023a-04af-4e9e-a3bc-a0db884a9339" xlink:to="loc_dei_EntityEmergingGrowthCompany_688e4fc6-ca73-4a49-984a-50e83a922026" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityExTransitionPeriod_c881d90a-5a5e-4b9a-a008-a4fb11a0197d" 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_32a0023a-04af-4e9e-a3bc-a0db884a9339" xlink:to="loc_dei_EntityExTransitionPeriod_c881d90a-5a5e-4b9a-a008-a4fb11a0197d" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityShellCompany_15aec468-32e9-4557-9c5a-2e3761ce84a0" 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_32a0023a-04af-4e9e-a3bc-a0db884a9339" xlink:to="loc_dei_EntityShellCompany_15aec468-32e9-4557-9c5a-2e3761ce84a0" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCommonStockSharesOutstanding_cb3ac6c1-d808-4195-94a3-97c2448ca1c7" 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_32a0023a-04af-4e9e-a3bc-a0db884a9339" xlink:to="loc_dei_EntityCommonStockSharesOutstanding_cb3ac6c1-d808-4195-94a3-97c2448ca1c7" xlink:type="arc" order="23"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_AmendmentFlag_c1da9ef7-099e-4724-b910-57b770d188d9" 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_32a0023a-04af-4e9e-a3bc-a0db884a9339" xlink:to="loc_dei_AmendmentFlag_c1da9ef7-099e-4724-b910-57b770d188d9" xlink:type="arc" order="24"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCentralIndexKey_6b1bce93-0b81-4201-93bd-94e6c49a3030" 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_32a0023a-04af-4e9e-a3bc-a0db884a9339" xlink:to="loc_dei_EntityCentralIndexKey_6b1bce93-0b81-4201-93bd-94e6c49a3030" xlink:type="arc" order="25"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_CurrentFiscalYearEndDate_da99105f-bb35-4801-a039-b98372a68efe" 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_32a0023a-04af-4e9e-a3bc-a0db884a9339" xlink:to="loc_dei_CurrentFiscalYearEndDate_da99105f-bb35-4801-a039-b98372a68efe" xlink:type="arc" order="26"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentFiscalYearFocus_53a52382-118d-44dc-8777-9df11b2e8dcd" 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_32a0023a-04af-4e9e-a3bc-a0db884a9339" xlink:to="loc_dei_DocumentFiscalYearFocus_53a52382-118d-44dc-8777-9df11b2e8dcd" xlink:type="arc" order="27"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentFiscalPeriodFocus_7a18db2e-ee55-4088-99b7-2b2858073212" 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_32a0023a-04af-4e9e-a3bc-a0db884a9339" xlink:to="loc_dei_DocumentFiscalPeriodFocus_7a18db2e-ee55-4088-99b7-2b2858073212" xlink:type="arc" order="28"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentInformationTable_0a7187ac-8999-4021-9428-684de1f8fbdf" 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_32a0023a-04af-4e9e-a3bc-a0db884a9339" xlink:to="loc_dei_DocumentInformationTable_0a7187ac-8999-4021-9428-684de1f8fbdf" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_75dbfa13-cfc6-4ecc-8558-ce2adf4b2e23" 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_0a7187ac-8999-4021-9428-684de1f8fbdf" xlink:to="loc_us-gaap_StatementClassOfStockAxis_75dbfa13-cfc6-4ecc-8558-ce2adf4b2e23" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_75dbfa13-cfc6-4ecc-8558-ce2adf4b2e23_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_75dbfa13-cfc6-4ecc-8558-ce2adf4b2e23" xlink:to="loc_us-gaap_ClassOfStockDomain_75dbfa13-cfc6-4ecc-8558-ce2adf4b2e23_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_f77782b0-1172-4fa7-b5b6-07056cb8e6e6" 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_75dbfa13-cfc6-4ecc-8558-ce2adf4b2e23" xlink:to="loc_us-gaap_ClassOfStockDomain_f77782b0-1172-4fa7-b5b6-07056cb8e6e6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_fcb0cc3b-c818-4ded-8773-3f7b9518b24c" 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_f77782b0-1172-4fa7-b5b6-07056cb8e6e6" xlink:to="loc_us-gaap_CommonClassAMember_fcb0cc3b-c818-4ded-8773-3f7b9518b24c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_4ec84806-09da-45a1-8b23-fbfdd86e8ed9" 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_f77782b0-1172-4fa7-b5b6-07056cb8e6e6" xlink:to="loc_us-gaap_CommonClassBMember_4ec84806-09da-45a1-8b23-fbfdd86e8ed9" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://pubmatic.com/role/CONDENSEDCONSOLIDATEDBALANCESHEETSParenthetical" xlink:type="simple" xlink:href="pubm-20210630.xsd#CONDENSEDCONSOLIDATEDBALANCESHEETSParenthetical"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/CONDENSEDCONSOLIDATEDBALANCESHEETSParenthetical" xlink:type="extended" id="ibde76ff0c74a451d92543ee7473918a9_CONDENSEDCONSOLIDATEDBALANCESHEETSParenthetical">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_d1c264b3-12bc-46b4-b961-b322e59213af" 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_64828199-2248-4306-9a79-15441146afbf" 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_d1c264b3-12bc-46b4-b961-b322e59213af" xlink:to="loc_us-gaap_PreferredStockParOrStatedValuePerShare_64828199-2248-4306-9a79-15441146afbf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesAuthorized_b204fd4d-add6-4672-8989-044d6ebd9006" 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_d1c264b3-12bc-46b4-b961-b322e59213af" xlink:to="loc_us-gaap_PreferredStockSharesAuthorized_b204fd4d-add6-4672-8989-044d6ebd9006" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesIssued_02a2246b-0dd9-42c4-a1f1-2115dc03ea39" 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_d1c264b3-12bc-46b4-b961-b322e59213af" xlink:to="loc_us-gaap_PreferredStockSharesIssued_02a2246b-0dd9-42c4-a1f1-2115dc03ea39" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesOutstanding_ef2b0fc7-5254-4267-b9bd-38e69400676e" 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_d1c264b3-12bc-46b4-b961-b322e59213af" xlink:to="loc_us-gaap_PreferredStockSharesOutstanding_ef2b0fc7-5254-4267-b9bd-38e69400676e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_ab83922b-894e-4265-a9c6-30d9cbd0fa2c" 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_d1c264b3-12bc-46b4-b961-b322e59213af" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_ab83922b-894e-4265-a9c6-30d9cbd0fa2c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesAuthorized_1f5a7d83-c15b-4587-95bb-d4b18b03e94a" 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_d1c264b3-12bc-46b4-b961-b322e59213af" xlink:to="loc_us-gaap_CommonStockSharesAuthorized_1f5a7d83-c15b-4587-95bb-d4b18b03e94a" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesIssued_1cfe91b8-27d0-43c9-852d-4f87f7c65c94" 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_d1c264b3-12bc-46b4-b961-b322e59213af" xlink:to="loc_us-gaap_CommonStockSharesIssued_1cfe91b8-27d0-43c9-852d-4f87f7c65c94" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_7941d285-4a1c-42ae-834b-75e24dbd2a08" 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_d1c264b3-12bc-46b4-b961-b322e59213af" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_7941d285-4a1c-42ae-834b-75e24dbd2a08" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockShares_3d7a386c-f29c-4284-a204-48680c003b51" 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_d1c264b3-12bc-46b4-b961-b322e59213af" xlink:to="loc_us-gaap_TreasuryStockShares_3d7a386c-f29c-4284-a204-48680c003b51" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_0a784ec9-bab2-40f9-857e-d595ca1c1595" 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_d1c264b3-12bc-46b4-b961-b322e59213af" xlink:to="loc_us-gaap_StatementTable_0a784ec9-bab2-40f9-857e-d595ca1c1595" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_78f2ac99-7d64-485e-b5e0-3d73a0f843bd" 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_0a784ec9-bab2-40f9-857e-d595ca1c1595" xlink:to="loc_us-gaap_StatementClassOfStockAxis_78f2ac99-7d64-485e-b5e0-3d73a0f843bd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_78f2ac99-7d64-485e-b5e0-3d73a0f843bd_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_78f2ac99-7d64-485e-b5e0-3d73a0f843bd" xlink:to="loc_us-gaap_ClassOfStockDomain_78f2ac99-7d64-485e-b5e0-3d73a0f843bd_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_20e4a6a0-b734-46ef-8794-4f5aaf4b8ab3" 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_78f2ac99-7d64-485e-b5e0-3d73a0f843bd" xlink:to="loc_us-gaap_ClassOfStockDomain_20e4a6a0-b734-46ef-8794-4f5aaf4b8ab3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_dc804122-0f7e-4ef9-ba2b-ab91ec1f0b60" 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_20e4a6a0-b734-46ef-8794-4f5aaf4b8ab3" xlink:to="loc_us-gaap_CommonClassAMember_dc804122-0f7e-4ef9-ba2b-ab91ec1f0b60" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_39512700-e959-405f-9203-6c7689a9ad4a" 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_20e4a6a0-b734-46ef-8794-4f5aaf4b8ab3" xlink:to="loc_us-gaap_CommonClassBMember_39512700-e959-405f-9203-6c7689a9ad4a" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://pubmatic.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITY" xlink:type="simple" xlink:href="pubm-20210630.xsd#CONDENSEDCONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITY"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITY" xlink:type="extended" id="i00aab7ae0c424e198f3a773548ddced9_CONDENSEDCONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITY">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_e95c3e61-b482-4511-8eee-1859c0422fc7" 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_f6c4478e-efba-45e8-baec-72ad9570a860" 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_e95c3e61-b482-4511-8eee-1859c0422fc7" xlink:to="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_f6c4478e-efba-45e8-baec-72ad9570a860" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquitySharesOutstanding_a309360e-53d7-43ef-b7d3-3949a2771fe4" 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_f6c4478e-efba-45e8-baec-72ad9570a860" xlink:to="loc_us-gaap_TemporaryEquitySharesOutstanding_a309360e-53d7-43ef-b7d3-3949a2771fe4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquityCarryingAmountAttributableToParent_2488cd78-2ffb-47fa-82b6-ed7242468060" 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_f6c4478e-efba-45e8-baec-72ad9570a860" xlink:to="loc_us-gaap_TemporaryEquityCarryingAmountAttributableToParent_2488cd78-2ffb-47fa-82b6-ed7242468060" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquitySharesOutstanding_0d6b7e31-2f24-4392-8b09-0e7ca6653e97" 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_4b9005f0-6536-41a0-8386-64d7feb94056" 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_6b1ba5d3-0742-4b97-957f-c3baf6dee7cd" 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_e95c3e61-b482-4511-8eee-1859c0422fc7" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_6b1ba5d3-0742-4b97-957f-c3baf6dee7cd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharesOutstanding_f176affb-4bcb-4aa4-9276-ddec75711f8d" 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_6b1ba5d3-0742-4b97-957f-c3baf6dee7cd" xlink:to="loc_us-gaap_SharesOutstanding_f176affb-4bcb-4aa4-9276-ddec75711f8d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_ef85301b-a553-4371-899a-7acad258a877" 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_6b1ba5d3-0742-4b97-957f-c3baf6dee7cd" xlink:to="loc_us-gaap_StockholdersEquity_ef85301b-a553-4371-899a-7acad258a877" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_15ba3025-8814-476c-9779-7074eae9827e" 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_6b1ba5d3-0742-4b97-957f-c3baf6dee7cd" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_15ba3025-8814-476c-9779-7074eae9827e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_252ac29f-ee15-4697-85be-2927a5782e17" 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_6b1ba5d3-0742-4b97-957f-c3baf6dee7cd" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_252ac29f-ee15-4697-85be-2927a5782e17" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_33ba94f5-0edb-4ff2-a1f3-2bb3bd2c26b2" 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_6b1ba5d3-0742-4b97-957f-c3baf6dee7cd" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_33ba94f5-0edb-4ff2-a1f3-2bb3bd2c26b2" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockSharesAcquired_ec212fa4-1393-47a4-96ec-c03c2e2c8e5a" 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_6b1ba5d3-0742-4b97-957f-c3baf6dee7cd" xlink:to="loc_us-gaap_TreasuryStockSharesAcquired_ec212fa4-1393-47a4-96ec-c03c2e2c8e5a" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockValueAcquiredCostMethod_3bc113e2-e44b-4d91-8c61-780de2ecb64c" 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_6b1ba5d3-0742-4b97-957f-c3baf6dee7cd" xlink:to="loc_us-gaap_TreasuryStockValueAcquiredCostMethod_3bc113e2-e44b-4d91-8c61-780de2ecb64c" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_9f917a75-bca5-4541-864c-7cea212e173b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_6b1ba5d3-0742-4b97-957f-c3baf6dee7cd" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_9f917a75-bca5-4541-864c-7cea212e173b" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan_220bc625-a7c6-438e-a41d-6a08daa1bca3" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_6b1ba5d3-0742-4b97-957f-c3baf6dee7cd" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan_220bc625-a7c6-438e-a41d-6a08daa1bca3" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_d4e5e27f-d2e7-4764-a4cc-9be4c97ae237" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesNewIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_6b1ba5d3-0742-4b97-957f-c3baf6dee7cd" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_d4e5e27f-d2e7-4764-a4cc-9be4c97ae237" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_2cd72e07-1e12-4856-91bc-dba4e47f1312" 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_6b1ba5d3-0742-4b97-957f-c3baf6dee7cd" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_2cd72e07-1e12-4856-91bc-dba4e47f1312" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_e6d4e9a2-b188-4389-8b75-b1404bb83740" 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_6b1ba5d3-0742-4b97-957f-c3baf6dee7cd" xlink:to="loc_us-gaap_NetIncomeLoss_e6d4e9a2-b188-4389-8b75-b1404bb83740" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharesOutstanding_b073bbba-e01f-4c0b-971a-6a30a4f94022" 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_9239caf2-56a8-4121-9a1e-d50955d6a2ae" 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_6a3c0562-bbee-4272-9531-a19bc0676c92" 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_e95c3e61-b482-4511-8eee-1859c0422fc7" xlink:to="loc_us-gaap_StatementTable_6a3c0562-bbee-4272-9531-a19bc0676c92" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_1c630799-6bb4-4595-b670-93166a704281" 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_6a3c0562-bbee-4272-9531-a19bc0676c92" xlink:to="loc_us-gaap_StatementClassOfStockAxis_1c630799-6bb4-4595-b670-93166a704281" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_1c630799-6bb4-4595-b670-93166a704281_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_1c630799-6bb4-4595-b670-93166a704281" xlink:to="loc_us-gaap_ClassOfStockDomain_1c630799-6bb4-4595-b670-93166a704281_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_280ad25f-9756-4df8-9dd1-3301653dcb8c" 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_1c630799-6bb4-4595-b670-93166a704281" xlink:to="loc_us-gaap_ClassOfStockDomain_280ad25f-9756-4df8-9dd1-3301653dcb8c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertiblePreferredStockMember_5c54f8c9-4e39-46e2-9f54-c903ac27c25f" 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_280ad25f-9756-4df8-9dd1-3301653dcb8c" xlink:to="loc_us-gaap_ConvertiblePreferredStockMember_5c54f8c9-4e39-46e2-9f54-c903ac27c25f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_pubm_RedeemableCommonStockMember_23c7445b-8700-47ee-9e1a-6be5c2f75f26" xlink:href="pubm-20210630.xsd#pubm_RedeemableCommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_280ad25f-9756-4df8-9dd1-3301653dcb8c" xlink:to="loc_pubm_RedeemableCommonStockMember_23c7445b-8700-47ee-9e1a-6be5c2f75f26" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_699e3b2e-3b47-47d5-a1a0-3acf10d201ed" 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_6a3c0562-bbee-4272-9531-a19bc0676c92" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_699e3b2e-3b47-47d5-a1a0-3acf10d201ed" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_699e3b2e-3b47-47d5-a1a0-3acf10d201ed_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_699e3b2e-3b47-47d5-a1a0-3acf10d201ed" xlink:to="loc_us-gaap_EquityComponentDomain_699e3b2e-3b47-47d5-a1a0-3acf10d201ed_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_ac4a6401-f2bc-45e1-8e41-f4690f346e0e" 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_699e3b2e-3b47-47d5-a1a0-3acf10d201ed" xlink:to="loc_us-gaap_EquityComponentDomain_ac4a6401-f2bc-45e1-8e41-f4690f346e0e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_1197cbf9-dcea-4e9f-9479-fbd8dfa1a39c" 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_ac4a6401-f2bc-45e1-8e41-f4690f346e0e" xlink:to="loc_us-gaap_CommonStockMember_1197cbf9-dcea-4e9f-9479-fbd8dfa1a39c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockMember_8345e8cf-3109-4b82-9af2-fe84c37491f1" 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_ac4a6401-f2bc-45e1-8e41-f4690f346e0e" xlink:to="loc_us-gaap_TreasuryStockMember_8345e8cf-3109-4b82-9af2-fe84c37491f1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_cfe7486c-0e14-405c-ad6c-3b686f7c828d" 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_ac4a6401-f2bc-45e1-8e41-f4690f346e0e" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_cfe7486c-0e14-405c-ad6c-3b686f7c828d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_e8f0466c-305c-4126-9396-6da00c8c405c" 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_ac4a6401-f2bc-45e1-8e41-f4690f346e0e" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_e8f0466c-305c-4126-9396-6da00c8c405c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_289e2197-69f4-41a5-898c-5e3b27634500" 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_ac4a6401-f2bc-45e1-8e41-f4690f346e0e" xlink:to="loc_us-gaap_RetainedEarningsMember_289e2197-69f4-41a5-898c-5e3b27634500" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://pubmatic.com/role/BasisofPresentationandSummaryofSignificantAccountingPoliciesInitialPublicOfferingDetails" xlink:type="simple" xlink:href="pubm-20210630.xsd#BasisofPresentationandSummaryofSignificantAccountingPoliciesInitialPublicOfferingDetails"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/BasisofPresentationandSummaryofSignificantAccountingPoliciesInitialPublicOfferingDetails" xlink:type="extended" id="i5e45608d3f684b94ba05fbb02b750aa3_BasisofPresentationandSummaryofSignificantAccountingPoliciesInitialPublicOfferingDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_3e25369d-27a9-4607-bf1a-d64b79fb8dd0" 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_dd00db56-6a93-4306-9198-fcd21a22010d" 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_3e25369d-27a9-4607-bf1a-d64b79fb8dd0" xlink:to="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_dd00db56-6a93-4306-9198-fcd21a22010d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockPricePerShare_fc36eaf7-a699-44e0-b2f1-06e13b3eb4de" 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_3e25369d-27a9-4607-bf1a-d64b79fb8dd0" xlink:to="loc_us-gaap_SaleOfStockPricePerShare_fc36eaf7-a699-44e0-b2f1-06e13b3eb4de" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_924be85a-56f9-4bc5-8ce5-9e77c9cbfa5d" 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_3e25369d-27a9-4607-bf1a-d64b79fb8dd0" xlink:to="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_924be85a-56f9-4bc5-8ce5-9e77c9cbfa5d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion_5130c30d-b427-486f-8791-907ea9adab59" 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_3e25369d-27a9-4607-bf1a-d64b79fb8dd0" xlink:to="loc_us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion_5130c30d-b427-486f-8791-907ea9adab59" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredOfferingCosts_294450ea-70bd-4351-9205-815285506333" 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_3e25369d-27a9-4607-bf1a-d64b79fb8dd0" xlink:to="loc_us-gaap_DeferredOfferingCosts_294450ea-70bd-4351-9205-815285506333" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEquityMethodInvestmentsTable_8d243ba7-c058-4563-a440-83891146ca73" 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_3e25369d-27a9-4607-bf1a-d64b79fb8dd0" xlink:to="loc_us-gaap_ScheduleOfEquityMethodInvestmentsTable_8d243ba7-c058-4563-a440-83891146ca73" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_c673c847-5124-4693-a084-2e36e426cb85" 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_8d243ba7-c058-4563-a440-83891146ca73" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_c673c847-5124-4693-a084-2e36e426cb85" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_c673c847-5124-4693-a084-2e36e426cb85_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_c673c847-5124-4693-a084-2e36e426cb85" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_c673c847-5124-4693-a084-2e36e426cb85_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_20089a4b-5f49-4946-863f-146a0144c029" 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_c673c847-5124-4693-a084-2e36e426cb85" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_20089a4b-5f49-4946-863f-146a0144c029" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IPOMember_fabf6fed-eaea-4541-8196-a60dcd9825be" 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_20089a4b-5f49-4946-863f-146a0144c029" xlink:to="loc_us-gaap_IPOMember_fabf6fed-eaea-4541-8196-a60dcd9825be" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_ca5aa804-7bcc-4fd7-a5db-36891d332527" 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_8d243ba7-c058-4563-a440-83891146ca73" xlink:to="loc_us-gaap_StatementClassOfStockAxis_ca5aa804-7bcc-4fd7-a5db-36891d332527" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_ca5aa804-7bcc-4fd7-a5db-36891d332527_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_ca5aa804-7bcc-4fd7-a5db-36891d332527" xlink:to="loc_us-gaap_ClassOfStockDomain_ca5aa804-7bcc-4fd7-a5db-36891d332527_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_362da839-9aab-4e64-a056-2416c5b602ef" 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_ca5aa804-7bcc-4fd7-a5db-36891d332527" xlink:to="loc_us-gaap_ClassOfStockDomain_362da839-9aab-4e64-a056-2416c5b602ef" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_b7e4f1de-af21-44f4-b8d8-1f20ce13e030" 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_362da839-9aab-4e64-a056-2416c5b602ef" xlink:to="loc_us-gaap_CommonClassBMember_b7e4f1de-af21-44f4-b8d8-1f20ce13e030" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://pubmatic.com/role/BasisofPresentationandSummaryofSignificantAccountingPoliciesConcentrationRiskPercentageDetails" xlink:type="simple" xlink:href="pubm-20210630.xsd#BasisofPresentationandSummaryofSignificantAccountingPoliciesConcentrationRiskPercentageDetails"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/BasisofPresentationandSummaryofSignificantAccountingPoliciesConcentrationRiskPercentageDetails" xlink:type="extended" id="i07b158691bf04ae1b74d45f8705513a6_BasisofPresentationandSummaryofSignificantAccountingPoliciesConcentrationRiskPercentageDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskLineItems_9f59305e-88af-4106-a489-d20cbcdc19fc" 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_46d97d49-82c2-4091-935f-29a0cacd8627" 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_9f59305e-88af-4106-a489-d20cbcdc19fc" xlink:to="loc_us-gaap_ConcentrationRiskPercentage1_46d97d49-82c2-4091-935f-29a0cacd8627" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTable_75492d6b-45a3-46ec-8315-ac503ecd3eb9" 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_9f59305e-88af-4106-a489-d20cbcdc19fc" xlink:to="loc_us-gaap_ConcentrationRiskTable_75492d6b-45a3-46ec-8315-ac503ecd3eb9" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByTypeAxis_93ac148c-4c8e-408d-b569-d2c4adae1bf6" 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_75492d6b-45a3-46ec-8315-ac503ecd3eb9" xlink:to="loc_us-gaap_ConcentrationRiskByTypeAxis_93ac148c-4c8e-408d-b569-d2c4adae1bf6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_93ac148c-4c8e-408d-b569-d2c4adae1bf6_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_93ac148c-4c8e-408d-b569-d2c4adae1bf6" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_93ac148c-4c8e-408d-b569-d2c4adae1bf6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_c1924fd8-c0b0-479f-b5be-ce7cc1e65516" 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_93ac148c-4c8e-408d-b569-d2c4adae1bf6" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_c1924fd8-c0b0-479f-b5be-ce7cc1e65516" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerConcentrationRiskMember_43280893-8847-48b3-be97-45b3e2dbb1bd" 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_c1924fd8-c0b0-479f-b5be-ce7cc1e65516" xlink:to="loc_us-gaap_CustomerConcentrationRiskMember_43280893-8847-48b3-be97-45b3e2dbb1bd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_ac7bb3e0-2964-438d-b5ed-6b9c543e2508" 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_75492d6b-45a3-46ec-8315-ac503ecd3eb9" xlink:to="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_ac7bb3e0-2964-438d-b5ed-6b9c543e2508" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_ac7bb3e0-2964-438d-b5ed-6b9c543e2508_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_ac7bb3e0-2964-438d-b5ed-6b9c543e2508" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_ac7bb3e0-2964-438d-b5ed-6b9c543e2508_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_e303d200-9f39-4bc5-abdd-84f8e48141d7" 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_ac7bb3e0-2964-438d-b5ed-6b9c543e2508" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_e303d200-9f39-4bc5-abdd-84f8e48141d7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsReceivableMember_3d804084-03ce-470d-902c-6ae7506b39a6" 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_e303d200-9f39-4bc5-abdd-84f8e48141d7" xlink:to="loc_us-gaap_AccountsReceivableMember_3d804084-03ce-470d-902c-6ae7506b39a6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MajorCustomersAxis_68d258a6-781c-4f39-b708-cf888d7576fc" 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_75492d6b-45a3-46ec-8315-ac503ecd3eb9" xlink:to="loc_srt_MajorCustomersAxis_68d258a6-781c-4f39-b708-cf888d7576fc" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_NameOfMajorCustomerDomain_68d258a6-781c-4f39-b708-cf888d7576fc_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_68d258a6-781c-4f39-b708-cf888d7576fc" xlink:to="loc_srt_NameOfMajorCustomerDomain_68d258a6-781c-4f39-b708-cf888d7576fc_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_NameOfMajorCustomerDomain_d51d2a8e-e3ef-434a-8b1e-9eb21cb07f06" 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_68d258a6-781c-4f39-b708-cf888d7576fc" xlink:to="loc_srt_NameOfMajorCustomerDomain_d51d2a8e-e3ef-434a-8b1e-9eb21cb07f06" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_pubm_OnePublisherMember_d4accd8d-30d6-4816-9fe2-dbd5040b540c" xlink:href="pubm-20210630.xsd#pubm_OnePublisherMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_d51d2a8e-e3ef-434a-8b1e-9eb21cb07f06" xlink:to="loc_pubm_OnePublisherMember_d4accd8d-30d6-4816-9fe2-dbd5040b540c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_pubm_BuyerOneMember_a47a32a7-dba2-4d80-adc0-944dcc5fa784" xlink:href="pubm-20210630.xsd#pubm_BuyerOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_d51d2a8e-e3ef-434a-8b1e-9eb21cb07f06" xlink:to="loc_pubm_BuyerOneMember_a47a32a7-dba2-4d80-adc0-944dcc5fa784" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_pubm_BuyerTwoMember_d7916631-b404-458c-b35b-05d041fa509a" xlink:href="pubm-20210630.xsd#pubm_BuyerTwoMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_d51d2a8e-e3ef-434a-8b1e-9eb21cb07f06" xlink:to="loc_pubm_BuyerTwoMember_d7916631-b404-458c-b35b-05d041fa509a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_pubm_BuyerThreeMember_8d466e38-3fe6-4da8-b8cc-3ddaac062d32" xlink:href="pubm-20210630.xsd#pubm_BuyerThreeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_d51d2a8e-e3ef-434a-8b1e-9eb21cb07f06" xlink:to="loc_pubm_BuyerThreeMember_8d466e38-3fe6-4da8-b8cc-3ddaac062d32" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_pubm_BuyerFourMember_087f6515-0ea9-4db3-850a-c2be99812319" xlink:href="pubm-20210630.xsd#pubm_BuyerFourMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_d51d2a8e-e3ef-434a-8b1e-9eb21cb07f06" xlink:to="loc_pubm_BuyerFourMember_087f6515-0ea9-4db3-850a-c2be99812319" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://pubmatic.com/role/FairValueMeasurementsDetails" xlink:type="simple" xlink:href="pubm-20210630.xsd#FairValueMeasurementsDetails"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/FairValueMeasurementsDetails" xlink:type="extended" id="ib1c8491bfd944da19a840bfe18a4c3c7_FairValueMeasurementsDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_60182b3f-c614-4b83-8760-f757d5d5fc8f" 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_a48fc02f-449e-4aec-bbff-8d3c35cf899d" 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_60182b3f-c614-4b83-8760-f757d5d5fc8f" xlink:to="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_a48fc02f-449e-4aec-bbff-8d3c35cf899d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_0ecb8f5c-2aef-439f-81c6-42e636648cb7" 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_60182b3f-c614-4b83-8760-f757d5d5fc8f" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_0ecb8f5c-2aef-439f-81c6-42e636648cb7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsFairValueDisclosure_963e606c-ba76-4ecc-8088-469f9e8277c2" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AssetsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_60182b3f-c614-4b83-8760-f757d5d5fc8f" xlink:to="loc_us-gaap_AssetsFairValueDisclosure_963e606c-ba76-4ecc-8088-469f9e8277c2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_f1fbb57f-2e30-4370-89e7-eb6b6a7f45ed" 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_60182b3f-c614-4b83-8760-f757d5d5fc8f" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_f1fbb57f-2e30-4370-89e7-eb6b6a7f45ed" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_d4b6d531-eade-4ac6-b247-028b00d40540" 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_f1fbb57f-2e30-4370-89e7-eb6b6a7f45ed" xlink:to="loc_us-gaap_FinancialInstrumentAxis_d4b6d531-eade-4ac6-b247-028b00d40540" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_d4b6d531-eade-4ac6-b247-028b00d40540_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_d4b6d531-eade-4ac6-b247-028b00d40540" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_d4b6d531-eade-4ac6-b247-028b00d40540_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_01efebe5-8576-4a7e-b8cb-950200b7869d" 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_d4b6d531-eade-4ac6-b247-028b00d40540" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_01efebe5-8576-4a7e-b8cb-950200b7869d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USTreasuryAndGovernmentMember_27a16665-8e5c-4e2f-a466-9e37113994fe" 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_01efebe5-8576-4a7e-b8cb-950200b7869d" xlink:to="loc_us-gaap_USTreasuryAndGovernmentMember_27a16665-8e5c-4e2f-a466-9e37113994fe" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialPaperMember_e87af0b2-80b5-4c66-a8ed-ea71ae665040" 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_01efebe5-8576-4a7e-b8cb-950200b7869d" xlink:to="loc_us-gaap_CommercialPaperMember_e87af0b2-80b5-4c66-a8ed-ea71ae665040" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_c8585ee4-0b94-4fa5-966f-1f7248c84428" 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_f1fbb57f-2e30-4370-89e7-eb6b6a7f45ed" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_c8585ee4-0b94-4fa5-966f-1f7248c84428" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_c8585ee4-0b94-4fa5-966f-1f7248c84428_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_c8585ee4-0b94-4fa5-966f-1f7248c84428" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_c8585ee4-0b94-4fa5-966f-1f7248c84428_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_af5bfacc-abed-4337-8ecf-5b6dbcdfdc1c" 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_c8585ee4-0b94-4fa5-966f-1f7248c84428" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_af5bfacc-abed-4337-8ecf-5b6dbcdfdc1c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_d0b3cc8f-ef42-49d4-b8d3-1aae010f8c3e" 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_af5bfacc-abed-4337-8ecf-5b6dbcdfdc1c" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_d0b3cc8f-ef42-49d4-b8d3-1aae010f8c3e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_98061872-d5fe-4e49-a443-6ed7afa23aea" 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_af5bfacc-abed-4337-8ecf-5b6dbcdfdc1c" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_98061872-d5fe-4e49-a443-6ed7afa23aea" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_e64245a9-14a2-4bb3-8e88-97fd11b5c6a0" 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_af5bfacc-abed-4337-8ecf-5b6dbcdfdc1c" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_e64245a9-14a2-4bb3-8e88-97fd11b5c6a0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAxis_f10713dc-5d0e-4509-b4b1-bcab2d1d501c" 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_f1fbb57f-2e30-4370-89e7-eb6b6a7f45ed" xlink:to="loc_us-gaap_CashAndCashEquivalentsAxis_f10713dc-5d0e-4509-b4b1-bcab2d1d501c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_f10713dc-5d0e-4509-b4b1-bcab2d1d501c_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_f10713dc-5d0e-4509-b4b1-bcab2d1d501c" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_f10713dc-5d0e-4509-b4b1-bcab2d1d501c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_04ec6261-6eb3-44a2-b59b-84677c6b31cf" 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_f10713dc-5d0e-4509-b4b1-bcab2d1d501c" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_04ec6261-6eb3-44a2-b59b-84677c6b31cf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MoneyMarketFundsMember_ca18c714-ddd8-413e-a206-3a2153564ea0" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_MoneyMarketFundsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_04ec6261-6eb3-44a2-b59b-84677c6b31cf" xlink:to="loc_us-gaap_MoneyMarketFundsMember_ca18c714-ddd8-413e-a206-3a2153564ea0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CertificatesOfDepositMember_b71ffe7a-b86c-4ad5-8883-237eb2938e1f" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CertificatesOfDepositMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_04ec6261-6eb3-44a2-b59b-84677c6b31cf" xlink:to="loc_us-gaap_CertificatesOfDepositMember_b71ffe7a-b86c-4ad5-8883-237eb2938e1f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashEquivalentsMember_1137f486-819f-448e-ae48-90195fbdec5d" 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_04ec6261-6eb3-44a2-b59b-84677c6b31cf" xlink:to="loc_us-gaap_CashEquivalentsMember_1137f486-819f-448e-ae48-90195fbdec5d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialPaperMember_78f19c48-9201-4783-acc0-b6137896d749" 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_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_04ec6261-6eb3-44a2-b59b-84677c6b31cf" xlink:to="loc_us-gaap_CommercialPaperMember_78f19c48-9201-4783-acc0-b6137896d749" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementFrequencyAxis_5da14ad7-00a3-4410-b101-1656926e0161" 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_f1fbb57f-2e30-4370-89e7-eb6b6a7f45ed" xlink:to="loc_us-gaap_FairValueByMeasurementFrequencyAxis_5da14ad7-00a3-4410-b101-1656926e0161" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_5da14ad7-00a3-4410-b101-1656926e0161_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_5da14ad7-00a3-4410-b101-1656926e0161" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_5da14ad7-00a3-4410-b101-1656926e0161_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_1703e803-0d57-4638-a8a4-9852746b2105" 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_5da14ad7-00a3-4410-b101-1656926e0161" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_1703e803-0d57-4638-a8a4-9852746b2105" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsRecurringMember_fd7afe1e-60ea-443a-b509-bcd15bb42651" 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_1703e803-0d57-4638-a8a4-9852746b2105" xlink:to="loc_us-gaap_FairValueMeasurementsRecurringMember_fd7afe1e-60ea-443a-b509-bcd15bb42651" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://pubmatic.com/role/BalanceSheetComponentsSummaryofMarketableSecuritiesDetails" xlink:type="simple" xlink:href="pubm-20210630.xsd#BalanceSheetComponentsSummaryofMarketableSecuritiesDetails"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/BalanceSheetComponentsSummaryofMarketableSecuritiesDetails" xlink:type="extended" id="ic38467fb96ff47be883a53cc5f1cb809_BalanceSheetComponentsSummaryofMarketableSecuritiesDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_77e868ca-8bd7-405c-914e-81301295074f" 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_c5eeed0a-65e2-46a8-98ae-2fac6be6062c" 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_77e868ca-8bd7-405c-914e-81301295074f" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostCurrent_c5eeed0a-65e2-46a8-98ae-2fac6be6062c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_903aa884-60de-4f64-b94d-756413533135" 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_77e868ca-8bd7-405c-914e-81301295074f" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_903aa884-60de-4f64-b94d-756413533135" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_83ecb2a4-2d34-44a2-90cd-98cdb468530d" 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_77e868ca-8bd7-405c-914e-81301295074f" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_83ecb2a4-2d34-44a2-90cd-98cdb468530d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecuritiesCurrent_e5f14353-9445-4000-a0f8-6fb6971df853" 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_77e868ca-8bd7-405c-914e-81301295074f" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecuritiesCurrent_e5f14353-9445-4000-a0f8-6fb6971df853" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_c15401ff-714d-4652-8b62-a162db54fb51" 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_77e868ca-8bd7-405c-914e-81301295074f" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_c15401ff-714d-4652-8b62-a162db54fb51" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_c7ea764c-47b0-4298-a645-2889a30b7087" 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_c15401ff-714d-4652-8b62-a162db54fb51" xlink:to="loc_us-gaap_FinancialInstrumentAxis_c7ea764c-47b0-4298-a645-2889a30b7087" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_c7ea764c-47b0-4298-a645-2889a30b7087_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_c7ea764c-47b0-4298-a645-2889a30b7087" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_c7ea764c-47b0-4298-a645-2889a30b7087_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_77f10ca6-c1dd-4464-b234-f9e80182d914" 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_c7ea764c-47b0-4298-a645-2889a30b7087" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_77f10ca6-c1dd-4464-b234-f9e80182d914" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialPaperMember_4fb6db12-e05c-4bed-8f07-6aa3580a4c21" 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_77f10ca6-c1dd-4464-b234-f9e80182d914" xlink:to="loc_us-gaap_CommercialPaperMember_4fb6db12-e05c-4bed-8f07-6aa3580a4c21" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USTreasuryAndGovernmentMember_65f3c758-7501-4f55-89fb-c2e6256923e6" 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_77f10ca6-c1dd-4464-b234-f9e80182d914" xlink:to="loc_us-gaap_USTreasuryAndGovernmentMember_65f3c758-7501-4f55-89fb-c2e6256923e6" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://pubmatic.com/role/BalanceSheetComponentsScheduleofPropertyEquipmentandSoftwareNetDetails" xlink:type="simple" xlink:href="pubm-20210630.xsd#BalanceSheetComponentsScheduleofPropertyEquipmentandSoftwareNetDetails"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/BalanceSheetComponentsScheduleofPropertyEquipmentandSoftwareNetDetails" xlink:type="extended" id="i3c4287558ccf4ab1ab876833a8f9c1f2_BalanceSheetComponentsScheduleofPropertyEquipmentandSoftwareNetDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_b7bf9acf-bcf4-42bb-bc0d-9d3053799197" 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_65996a27-7e19-4f65-a762-d113d8cfbfe4" 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_b7bf9acf-bcf4-42bb-bc0d-9d3053799197" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentGross_65996a27-7e19-4f65-a762-d113d8cfbfe4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_c930fbce-d989-4d00-97fe-5e16aa7dda46" 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_b7bf9acf-bcf4-42bb-bc0d-9d3053799197" xlink:to="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_c930fbce-d989-4d00-97fe-5e16aa7dda46" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet_b1d080a1-5e94-4f72-8699-9ebd255ec5af" 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_b7bf9acf-bcf4-42bb-bc0d-9d3053799197" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet_b1d080a1-5e94-4f72-8699-9ebd255ec5af" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_34acb856-4fa4-4e90-bc5b-1dfe63a5ed90" 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_b7bf9acf-bcf4-42bb-bc0d-9d3053799197" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_34acb856-4fa4-4e90-bc5b-1dfe63a5ed90" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_27f38705-23f0-4adc-a349-35ce4444c32b" 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_34acb856-4fa4-4e90-bc5b-1dfe63a5ed90" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_27f38705-23f0-4adc-a349-35ce4444c32b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_27f38705-23f0-4adc-a349-35ce4444c32b_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_27f38705-23f0-4adc-a349-35ce4444c32b" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_27f38705-23f0-4adc-a349-35ce4444c32b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_ef3e0a82-7fa7-437e-af00-0f7c2b1dc779" 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_27f38705-23f0-4adc-a349-35ce4444c32b" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_ef3e0a82-7fa7-437e-af00-0f7c2b1dc779" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SoftwareDevelopmentMember_abf4b80c-fde6-4502-8d63-532d6d87511f" 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_ef3e0a82-7fa7-437e-af00-0f7c2b1dc779" xlink:to="loc_us-gaap_SoftwareDevelopmentMember_abf4b80c-fde6-4502-8d63-532d6d87511f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TechnologyEquipmentMember_bbc45468-c561-411d-a42b-cec92b13aac3" 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_ef3e0a82-7fa7-437e-af00-0f7c2b1dc779" xlink:to="loc_us-gaap_TechnologyEquipmentMember_bbc45468-c561-411d-a42b-cec92b13aac3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseholdImprovementsMember_1e237664-52d3-4d95-bf3f-7814547d0cdd" 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_ef3e0a82-7fa7-437e-af00-0f7c2b1dc779" xlink:to="loc_us-gaap_LeaseholdImprovementsMember_1e237664-52d3-4d95-bf3f-7814547d0cdd" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FurnitureAndFixturesMember_1d6de92d-d9d1-47f2-ab87-52688180e9b6" 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_ef3e0a82-7fa7-437e-af00-0f7c2b1dc779" xlink:to="loc_us-gaap_FurnitureAndFixturesMember_1d6de92d-d9d1-47f2-ab87-52688180e9b6" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://pubmatic.com/role/BalanceSheetComponentsNarrativeDetails" xlink:type="simple" xlink:href="pubm-20210630.xsd#BalanceSheetComponentsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/BalanceSheetComponentsNarrativeDetails" xlink:type="extended" id="i9de190dac64b4ac98d1fdb54723e28db_BalanceSheetComponentsNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_b6c3120b-9299-4e7b-a04b-37f53719ae42" 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_be3ff541-c21e-4d2d-8d60-87314d1994fd" 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_b6c3120b-9299-4e7b-a04b-37f53719ae42" xlink:to="loc_us-gaap_DepreciationDepletionAndAmortization_be3ff541-c21e-4d2d-8d60-87314d1994fd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CapitalizedComputerSoftwareAdditions_64ad912d-991f-4d9a-b324-a24e22a7f099" 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_b6c3120b-9299-4e7b-a04b-37f53719ae42" xlink:to="loc_us-gaap_CapitalizedComputerSoftwareAdditions_64ad912d-991f-4d9a-b324-a24e22a7f099" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentForAmortization_7f0edd88-4a47-456d-bd48-3f5b39286779" 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_b6c3120b-9299-4e7b-a04b-37f53719ae42" xlink:to="loc_us-gaap_AdjustmentForAmortization_7f0edd88-4a47-456d-bd48-3f5b39286779" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_0ae3cf9e-9a0d-4e94-8bcd-c92a1a03b0c2" 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_b6c3120b-9299-4e7b-a04b-37f53719ae42" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_0ae3cf9e-9a0d-4e94-8bcd-c92a1a03b0c2" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_d4132a03-0722-45a4-bbb2-3ea504d859b3" 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_0ae3cf9e-9a0d-4e94-8bcd-c92a1a03b0c2" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_d4132a03-0722-45a4-bbb2-3ea504d859b3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_d4132a03-0722-45a4-bbb2-3ea504d859b3_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_d4132a03-0722-45a4-bbb2-3ea504d859b3" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_d4132a03-0722-45a4-bbb2-3ea504d859b3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_c6053b0e-a32a-4f18-b28a-574faf102373" 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_d4132a03-0722-45a4-bbb2-3ea504d859b3" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_c6053b0e-a32a-4f18-b28a-574faf102373" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SoftwareDevelopmentMember_2b8104b1-85f8-45ac-8ad8-2e6f5990ede4" 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_c6053b0e-a32a-4f18-b28a-574faf102373" xlink:to="loc_us-gaap_SoftwareDevelopmentMember_2b8104b1-85f8-45ac-8ad8-2e6f5990ede4" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://pubmatic.com/role/LoanandSecurityAgreementDetails" xlink:type="simple" xlink:href="pubm-20210630.xsd#LoanandSecurityAgreementDetails"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/LoanandSecurityAgreementDetails" xlink:type="extended" id="icdbaffdcb43d4ee69ea7de71f8f08302_LoanandSecurityAgreementDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityLineItems_ff8a38be-e302-46bd-b3f9-df2069e8264e" 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_cca25401-a6e3-4a1f-802a-59e9f490a766" 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_ff8a38be-e302-46bd-b3f9-df2069e8264e" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_cca25401-a6e3-4a1f-802a-59e9f490a766" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_pubm_LineOfCreditFacilityBorrowingCapacityPercentageOfEligibleAccountsReceivableNet_700355ee-f824-4157-b331-686501e533fb" xlink:href="pubm-20210630.xsd#pubm_LineOfCreditFacilityBorrowingCapacityPercentageOfEligibleAccountsReceivableNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_ff8a38be-e302-46bd-b3f9-df2069e8264e" xlink:to="loc_pubm_LineOfCreditFacilityBorrowingCapacityPercentageOfEligibleAccountsReceivableNet_700355ee-f824-4157-b331-686501e533fb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage_7562caa8-500e-468d-bcf7-d881cd575d18" 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_ff8a38be-e302-46bd-b3f9-df2069e8264e" xlink:to="loc_us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage_7562caa8-500e-468d-bcf7-d881cd575d18" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_pubm_LineOfCreditFacilityBorrowingCapacityUnusedCapacityFeeClosingBalanceThreshold_9a7da7d5-ef2e-423e-a1bc-963fe1d1613b" xlink:href="pubm-20210630.xsd#pubm_LineOfCreditFacilityBorrowingCapacityUnusedCapacityFeeClosingBalanceThreshold"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_ff8a38be-e302-46bd-b3f9-df2069e8264e" xlink:to="loc_pubm_LineOfCreditFacilityBorrowingCapacityUnusedCapacityFeeClosingBalanceThreshold_9a7da7d5-ef2e-423e-a1bc-963fe1d1613b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_ec25bcd0-de6a-4de3-b01e-b25a6da5d2bf" 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_ff8a38be-e302-46bd-b3f9-df2069e8264e" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_ec25bcd0-de6a-4de3-b01e-b25a6da5d2bf" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_pubm_DebtInstrumentCovenantQuickRatioMinimum_e80bd376-c19e-478e-9bd1-12fd3febdc2d" xlink:href="pubm-20210630.xsd#pubm_DebtInstrumentCovenantQuickRatioMinimum"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_ff8a38be-e302-46bd-b3f9-df2069e8264e" xlink:to="loc_pubm_DebtInstrumentCovenantQuickRatioMinimum_e80bd376-c19e-478e-9bd1-12fd3febdc2d" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityTable_f87f0f24-1a3a-443c-ac0a-0c1fc3bf37fc" 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_ff8a38be-e302-46bd-b3f9-df2069e8264e" xlink:to="loc_us-gaap_LineOfCreditFacilityTable_f87f0f24-1a3a-443c-ac0a-0c1fc3bf37fc" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_63f0cf5e-07f0-4b7f-a638-e1f48214b1ce" 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_f87f0f24-1a3a-443c-ac0a-0c1fc3bf37fc" xlink:to="loc_us-gaap_CreditFacilityAxis_63f0cf5e-07f0-4b7f-a638-e1f48214b1ce" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_63f0cf5e-07f0-4b7f-a638-e1f48214b1ce_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_63f0cf5e-07f0-4b7f-a638-e1f48214b1ce" xlink:to="loc_us-gaap_CreditFacilityDomain_63f0cf5e-07f0-4b7f-a638-e1f48214b1ce_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_a4f3a0c4-db96-479b-9cfd-f5c2fab9c5d3" 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_63f0cf5e-07f0-4b7f-a638-e1f48214b1ce" xlink:to="loc_us-gaap_CreditFacilityDomain_a4f3a0c4-db96-479b-9cfd-f5c2fab9c5d3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_b10029c6-d75d-4b5f-9eb1-0b244c0f9706" 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_a4f3a0c4-db96-479b-9cfd-f5c2fab9c5d3" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_b10029c6-d75d-4b5f-9eb1-0b244c0f9706" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityAxis_a4fe9a85-f670-4247-9433-74c9e238cac8" 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_f87f0f24-1a3a-443c-ac0a-0c1fc3bf37fc" xlink:to="loc_us-gaap_LineOfCreditFacilityAxis_a4fe9a85-f670-4247-9433-74c9e238cac8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityLenderDomain_a4fe9a85-f670-4247-9433-74c9e238cac8_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_a4fe9a85-f670-4247-9433-74c9e238cac8" xlink:to="loc_us-gaap_LineOfCreditFacilityLenderDomain_a4fe9a85-f670-4247-9433-74c9e238cac8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityLenderDomain_b3537a06-030c-46ad-8fc5-41e5e8827d4c" 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_a4fe9a85-f670-4247-9433-74c9e238cac8" xlink:to="loc_us-gaap_LineOfCreditFacilityLenderDomain_b3537a06-030c-46ad-8fc5-41e5e8827d4c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_pubm_SiliconValleyBankMember_d7e4a735-f701-4400-b0ce-940d085c491a" xlink:href="pubm-20210630.xsd#pubm_SiliconValleyBankMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLenderDomain_b3537a06-030c-46ad-8fc5-41e5e8827d4c" xlink:to="loc_pubm_SiliconValleyBankMember_d7e4a735-f701-4400-b0ce-940d085c491a" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://pubmatic.com/role/ConvertiblePreferredStockDetails" xlink:type="simple" xlink:href="pubm-20210630.xsd#ConvertiblePreferredStockDetails"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/ConvertiblePreferredStockDetails" xlink:type="extended" id="i8bc44204de454bc98f98c5dee30a5ea7_ConvertiblePreferredStockDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredUnitsLineItems_d17c32bc-7064-4487-8cfc-04866843c006" 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_3de1c4e8-635e-42d7-856a-3dac92678ed1" 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_d17c32bc-7064-4487-8cfc-04866843c006" xlink:to="loc_us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion_3de1c4e8-635e-42d7-856a-3dac92678ed1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquityCarryingAmountAttributableToParent_d2233f4c-efc1-40ab-81cc-dd9ed7a19e30" 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_d17c32bc-7064-4487-8cfc-04866843c006" xlink:to="loc_us-gaap_TemporaryEquityCarryingAmountAttributableToParent_d2233f4c-efc1-40ab-81cc-dd9ed7a19e30" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesAuthorized_c21e6fca-61fa-4d11-b77f-8fe3daf8050d" 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_d17c32bc-7064-4487-8cfc-04866843c006" xlink:to="loc_us-gaap_PreferredStockSharesAuthorized_c21e6fca-61fa-4d11-b77f-8fe3daf8050d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockParOrStatedValuePerShare_b55ba666-cbb9-460a-8f61-390ab29b716d" 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_d17c32bc-7064-4487-8cfc-04866843c006" xlink:to="loc_us-gaap_PreferredStockParOrStatedValuePerShare_b55ba666-cbb9-460a-8f61-390ab29b716d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfPreferredUnitsTable_ebd65251-1158-4cbe-a572-02a7155c820a" 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_d17c32bc-7064-4487-8cfc-04866843c006" xlink:to="loc_us-gaap_ScheduleOfPreferredUnitsTable_ebd65251-1158-4cbe-a572-02a7155c820a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_19a0c35a-b5f9-4150-98a1-85341f72c20d" 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_ebd65251-1158-4cbe-a572-02a7155c820a" xlink:to="loc_us-gaap_StatementClassOfStockAxis_19a0c35a-b5f9-4150-98a1-85341f72c20d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_19a0c35a-b5f9-4150-98a1-85341f72c20d_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_19a0c35a-b5f9-4150-98a1-85341f72c20d" xlink:to="loc_us-gaap_ClassOfStockDomain_19a0c35a-b5f9-4150-98a1-85341f72c20d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_ac9ac779-16c1-42aa-97fd-deb10bd2b8c4" 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_19a0c35a-b5f9-4150-98a1-85341f72c20d" xlink:to="loc_us-gaap_ClassOfStockDomain_ac9ac779-16c1-42aa-97fd-deb10bd2b8c4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_b6239ccc-bdca-4bd0-874d-09bae32c9567" 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_ac9ac779-16c1-42aa-97fd-deb10bd2b8c4" xlink:to="loc_us-gaap_CommonClassBMember_b6239ccc-bdca-4bd0-874d-09bae32c9567" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertiblePreferredStockMember_76a60be6-e50c-4faf-85c9-b83accc5fb79" 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_ac9ac779-16c1-42aa-97fd-deb10bd2b8c4" xlink:to="loc_us-gaap_ConvertiblePreferredStockMember_76a60be6-e50c-4faf-85c9-b83accc5fb79" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://pubmatic.com/role/StockholdersNotesReceivableandRedeemableCommonStockDetails" xlink:type="simple" xlink:href="pubm-20210630.xsd#StockholdersNotesReceivableandRedeemableCommonStockDetails"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/StockholdersNotesReceivableandRedeemableCommonStockDetails" xlink:type="extended" id="i149749130c3045dfb0ffe41f6f057d78_StockholdersNotesReceivableandRedeemableCommonStockDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockLineItems_ef97b5d8-6473-4154-a8ca-c9d633bac83c" 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_4aa2799e-e717-414b-a9d1-5c468f2b0016" 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_ef97b5d8-6473-4154-a8ca-c9d633bac83c" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_4aa2799e-e717-414b-a9d1-5c468f2b0016" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_cc8b6059-ebe9-4575-8753-7d232a1c0f6a" 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_ef97b5d8-6473-4154-a8ca-c9d633bac83c" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_cc8b6059-ebe9-4575-8753-7d232a1c0f6a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_pubm_TemporaryEquityCommonStockSharesIssued_53343199-7864-42b3-9b45-258fb85cc56f" xlink:href="pubm-20210630.xsd#pubm_TemporaryEquityCommonStockSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_ef97b5d8-6473-4154-a8ca-c9d633bac83c" xlink:to="loc_pubm_TemporaryEquityCommonStockSharesIssued_53343199-7864-42b3-9b45-258fb85cc56f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquityCarryingAmountAttributableToParent_ad4cbe4a-8fd6-4448-9e8b-7205baa4a562" 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_ef97b5d8-6473-4154-a8ca-c9d633bac83c" xlink:to="loc_us-gaap_TemporaryEquityCarryingAmountAttributableToParent_ad4cbe4a-8fd6-4448-9e8b-7205baa4a562" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_bf21978d-f855-41a6-8403-a19ac2a60e86" 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_ef97b5d8-6473-4154-a8ca-c9d633bac83c" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_bf21978d-f855-41a6-8403-a19ac2a60e86" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_e4a16679-db6a-4ab9-9a46-edb529afe6b6" 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_bf21978d-f855-41a6-8403-a19ac2a60e86" xlink:to="loc_us-gaap_StatementClassOfStockAxis_e4a16679-db6a-4ab9-9a46-edb529afe6b6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_e4a16679-db6a-4ab9-9a46-edb529afe6b6_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_e4a16679-db6a-4ab9-9a46-edb529afe6b6" xlink:to="loc_us-gaap_ClassOfStockDomain_e4a16679-db6a-4ab9-9a46-edb529afe6b6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_e68c3a24-7f64-458e-b37f-b98c4f07c6bd" 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_e4a16679-db6a-4ab9-9a46-edb529afe6b6" xlink:to="loc_us-gaap_ClassOfStockDomain_e68c3a24-7f64-458e-b37f-b98c4f07c6bd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_pubm_RedeemableCommonStockMember_c9f641aa-d244-4e11-883c-f79f8babb15c" xlink:href="pubm-20210630.xsd#pubm_RedeemableCommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_e68c3a24-7f64-458e-b37f-b98c4f07c6bd" xlink:to="loc_pubm_RedeemableCommonStockMember_c9f641aa-d244-4e11-883c-f79f8babb15c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_7cd272f1-7bea-4cf8-8246-62af68fd9658" 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_bf21978d-f855-41a6-8403-a19ac2a60e86" xlink:to="loc_us-gaap_DebtInstrumentAxis_7cd272f1-7bea-4cf8-8246-62af68fd9658" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_7cd272f1-7bea-4cf8-8246-62af68fd9658_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_7cd272f1-7bea-4cf8-8246-62af68fd9658" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_7cd272f1-7bea-4cf8-8246-62af68fd9658_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_c7bc17d8-e8ee-4d5c-adbe-f6c2f323ddfb" 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_7cd272f1-7bea-4cf8-8246-62af68fd9658" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_c7bc17d8-e8ee-4d5c-adbe-f6c2f323ddfb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_pubm_TwoPointFourTwoPercentPromissoryNotesDueAugust2021Member_5947df19-704e-4807-91bd-d755f949ca0d" xlink:href="pubm-20210630.xsd#pubm_TwoPointFourTwoPercentPromissoryNotesDueAugust2021Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_c7bc17d8-e8ee-4d5c-adbe-f6c2f323ddfb" xlink:to="loc_pubm_TwoPointFourTwoPercentPromissoryNotesDueAugust2021Member_5947df19-704e-4807-91bd-d755f949ca0d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_d9b2f697-c039-4ffe-a492-cd363aabacbb" 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_bf21978d-f855-41a6-8403-a19ac2a60e86" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_d9b2f697-c039-4ffe-a492-cd363aabacbb" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_d9b2f697-c039-4ffe-a492-cd363aabacbb_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_d9b2f697-c039-4ffe-a492-cd363aabacbb" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_d9b2f697-c039-4ffe-a492-cd363aabacbb_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_9340d082-e791-40d1-9eea-12cd9031b31c" 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_d9b2f697-c039-4ffe-a492-cd363aabacbb" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_9340d082-e791-40d1-9eea-12cd9031b31c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NotesPayableOtherPayablesMember_d89d51ac-9797-4db1-b390-f4b0db388c51" 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_9340d082-e791-40d1-9eea-12cd9031b31c" xlink:to="loc_us-gaap_NotesPayableOtherPayablesMember_d89d51ac-9797-4db1-b390-f4b0db388c51" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://pubmatic.com/role/StockholdersEquityandStockOptionPlansNarrativeDetails" xlink:type="simple" xlink:href="pubm-20210630.xsd#StockholdersEquityandStockOptionPlansNarrativeDetails"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/StockholdersEquityandStockOptionPlansNarrativeDetails" xlink:type="extended" id="i08fb6da4f1584a879f6d69c4c9fd4a56_StockholdersEquityandStockOptionPlansNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockLineItems_a51ef78a-ed28-457c-a6d0-3070e547e20e" 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_28c815de-70c9-4cf9-8527-03609be2ccb5" 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_a51ef78a-ed28-457c-a6d0-3070e547e20e" xlink:to="loc_us-gaap_CommonStockSharesAuthorized_28c815de-70c9-4cf9-8527-03609be2ccb5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_8585aabd-64aa-4790-8288-ec49839ef85e" 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_a51ef78a-ed28-457c-a6d0-3070e547e20e" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_8585aabd-64aa-4790-8288-ec49839ef85e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_6f0dc64b-5e8f-4c42-8779-8c8f224ed4a4" 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_a51ef78a-ed28-457c-a6d0-3070e547e20e" xlink:to="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_6f0dc64b-5e8f-4c42-8779-8c8f224ed4a4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_7261e829-b17e-43e0-881d-3fabef831c45" 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_a51ef78a-ed28-457c-a6d0-3070e547e20e" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_7261e829-b17e-43e0-881d-3fabef831c45" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_ae6e0afa-ce09-4db7-9be1-3023272e0fb5" 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_a51ef78a-ed28-457c-a6d0-3070e547e20e" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_ae6e0afa-ce09-4db7-9be1-3023272e0fb5" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_8e7b01f1-d476-4158-bf8c-981083035649" 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_a51ef78a-ed28-457c-a6d0-3070e547e20e" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_8e7b01f1-d476-4158-bf8c-981083035649" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentageOfOutstandingStockMaximum_19f6a34b-60cd-4311-b8d5-2d7238f6e64a" 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_a51ef78a-ed28-457c-a6d0-3070e547e20e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentageOfOutstandingStockMaximum_19f6a34b-60cd-4311-b8d5-2d7238f6e64a" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_4937c437-aa2c-4177-85de-b332dc15bfa1" 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_a51ef78a-ed28-457c-a6d0-3070e547e20e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_4937c437-aa2c-4177-85de-b332dc15bfa1" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardDiscountFromMarketPricePurchaseDate_9d793742-6ad0-4d7b-bdfe-ae0d726b19f2" 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_a51ef78a-ed28-457c-a6d0-3070e547e20e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardDiscountFromMarketPricePurchaseDate_9d793742-6ad0-4d7b-bdfe-ae0d726b19f2" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_pubm_ShareBasedCompensationArrangementByShareBasedPaymentAwardAmountWithheldForEmployeeStockPurchasePlan_521880e5-dcb7-4254-8cfb-2fa313dacadc" xlink:href="pubm-20210630.xsd#pubm_ShareBasedCompensationArrangementByShareBasedPaymentAwardAmountWithheldForEmployeeStockPurchasePlan"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_a51ef78a-ed28-457c-a6d0-3070e547e20e" xlink:to="loc_pubm_ShareBasedCompensationArrangementByShareBasedPaymentAwardAmountWithheldForEmployeeStockPurchasePlan_521880e5-dcb7-4254-8cfb-2fa313dacadc" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_5efa7476-3e90-4f0e-8553-036fc50c3712" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_a51ef78a-ed28-457c-a6d0-3070e547e20e" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_5efa7476-3e90-4f0e-8553-036fc50c3712" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_0f3546e6-770c-468b-83e5-7b871535a909" 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_a51ef78a-ed28-457c-a6d0-3070e547e20e" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_0f3546e6-770c-468b-83e5-7b871535a909" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_3f39c85a-c375-4ba4-a6a3-492d414d88c7" 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_0f3546e6-770c-468b-83e5-7b871535a909" xlink:to="loc_us-gaap_StatementClassOfStockAxis_3f39c85a-c375-4ba4-a6a3-492d414d88c7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_3f39c85a-c375-4ba4-a6a3-492d414d88c7_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_3f39c85a-c375-4ba4-a6a3-492d414d88c7" xlink:to="loc_us-gaap_ClassOfStockDomain_3f39c85a-c375-4ba4-a6a3-492d414d88c7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_fdec1ed3-36d3-413c-bd61-86cfb545dafa" 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_3f39c85a-c375-4ba4-a6a3-492d414d88c7" xlink:to="loc_us-gaap_ClassOfStockDomain_fdec1ed3-36d3-413c-bd61-86cfb545dafa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_e9f3e2b4-10fd-4d72-b424-29f881c6c49f" 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_fdec1ed3-36d3-413c-bd61-86cfb545dafa" xlink:to="loc_us-gaap_CommonClassAMember_e9f3e2b4-10fd-4d72-b424-29f881c6c49f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_52a540f7-4a3f-44d8-9ea9-cf8e5ff3fc6d" 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_fdec1ed3-36d3-413c-bd61-86cfb545dafa" xlink:to="loc_us-gaap_CommonClassBMember_52a540f7-4a3f-44d8-9ea9-cf8e5ff3fc6d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_1ee92d58-0c35-445c-b49c-aac612b4db9b" 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_0f3546e6-770c-468b-83e5-7b871535a909" xlink:to="loc_us-gaap_AwardTypeAxis_1ee92d58-0c35-445c-b49c-aac612b4db9b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_1ee92d58-0c35-445c-b49c-aac612b4db9b_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_1ee92d58-0c35-445c-b49c-aac612b4db9b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_1ee92d58-0c35-445c-b49c-aac612b4db9b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_db420b90-8ea8-4eb6-a3f2-5b28ab0815f7" 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_1ee92d58-0c35-445c-b49c-aac612b4db9b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_db420b90-8ea8-4eb6-a3f2-5b28ab0815f7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_652fd897-2098-4120-9e4a-182acf45c5e4" 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_db420b90-8ea8-4eb6-a3f2-5b28ab0815f7" xlink:to="loc_us-gaap_EmployeeStockOptionMember_652fd897-2098-4120-9e4a-182acf45c5e4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_f21c8de3-6c5e-4555-b6db-4323f770bcba" 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_db420b90-8ea8-4eb6-a3f2-5b28ab0815f7" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_f21c8de3-6c5e-4555-b6db-4323f770bcba" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember_9bb1dcb4-9830-406e-a5dc-ae279cb12cb3" 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_db420b90-8ea8-4eb6-a3f2-5b28ab0815f7" xlink:to="loc_us-gaap_EmployeeStockMember_9bb1dcb4-9830-406e-a5dc-ae279cb12cb3" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://pubmatic.com/role/StockholdersEquityandStockOptionPlansSummaryofRestrictedStockUnitsandRelatedInformationDetails" xlink:type="simple" xlink:href="pubm-20210630.xsd#StockholdersEquityandStockOptionPlansSummaryofRestrictedStockUnitsandRelatedInformationDetails"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/StockholdersEquityandStockOptionPlansSummaryofRestrictedStockUnitsandRelatedInformationDetails" xlink:type="extended" id="ic017013ad4b2461cba051d81d7c39186_StockholdersEquityandStockOptionPlansSummaryofRestrictedStockUnitsandRelatedInformationDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_02429682-40b9-4739-8ae4-306faef386da" 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_b34b86b9-8783-442c-b09f-cf63a9bbbc35" 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_02429682-40b9-4739-8ae4-306faef386da" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_b34b86b9-8783-442c-b09f-cf63a9bbbc35" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_9842c291-b51a-4929-a18f-6729ae827165" 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_b34b86b9-8783-442c-b09f-cf63a9bbbc35" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_9842c291-b51a-4929-a18f-6729ae827165" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_b4255c77-148e-4577-b3ea-a6f2f204b878" 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_b34b86b9-8783-442c-b09f-cf63a9bbbc35" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_b4255c77-148e-4577-b3ea-a6f2f204b878" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_ef7a4a6f-da12-4e88-9af3-546ef23b5817" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_b34b86b9-8783-442c-b09f-cf63a9bbbc35" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_ef7a4a6f-da12-4e88-9af3-546ef23b5817" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_fc0a6b35-52c4-4956-be32-ca2111149ecb" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_b34b86b9-8783-442c-b09f-cf63a9bbbc35" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_fc0a6b35-52c4-4956-be32-ca2111149ecb" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_00bc977e-f76f-4ad5-bb31-beaf779af936" 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_90f69b0f-b7ed-4de2-b64b-7c3ab36a08e9" 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_02429682-40b9-4739-8ae4-306faef386da" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_90f69b0f-b7ed-4de2-b64b-7c3ab36a08e9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_6937dc26-f253-4508-bb35-ad77f81ac24a" 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_90f69b0f-b7ed-4de2-b64b-7c3ab36a08e9" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_6937dc26-f253-4508-bb35-ad77f81ac24a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_2e0a3eda-fde5-4d41-ace4-62c698eebc64" 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_90f69b0f-b7ed-4de2-b64b-7c3ab36a08e9" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_2e0a3eda-fde5-4d41-ace4-62c698eebc64" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_0f593ff1-06eb-46a2-9b4c-0d7cfeef6b17" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_90f69b0f-b7ed-4de2-b64b-7c3ab36a08e9" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_0f593ff1-06eb-46a2-9b4c-0d7cfeef6b17" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_4630d4db-abe4-422b-a2d0-6105e33a473c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_90f69b0f-b7ed-4de2-b64b-7c3ab36a08e9" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_4630d4db-abe4-422b-a2d0-6105e33a473c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_b663e4c8-447e-411f-972d-fd48bf511c3d" 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_4536e79c-9bf9-4815-8fca-a799ddd8b9dc" 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_02429682-40b9-4739-8ae4-306faef386da" xlink:to="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_4536e79c-9bf9-4815-8fca-a799ddd8b9dc" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_8f990c66-f533-4767-83f7-7d401e093f7c" 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_4536e79c-9bf9-4815-8fca-a799ddd8b9dc" xlink:to="loc_us-gaap_AwardTypeAxis_8f990c66-f533-4767-83f7-7d401e093f7c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_8f990c66-f533-4767-83f7-7d401e093f7c_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_8f990c66-f533-4767-83f7-7d401e093f7c" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_8f990c66-f533-4767-83f7-7d401e093f7c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_db92c000-0907-4f4d-94a0-472db1c659dc" 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_8f990c66-f533-4767-83f7-7d401e093f7c" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_db92c000-0907-4f4d-94a0-472db1c659dc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_dcb35c91-a188-47f2-a616-d6355c85544d" 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_db92c000-0907-4f4d-94a0-472db1c659dc" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_dcb35c91-a188-47f2-a616-d6355c85544d" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://pubmatic.com/role/StockholdersEquityandStockOptionPlansStockbasedCompensationExpenseDetails" xlink:type="simple" xlink:href="pubm-20210630.xsd#StockholdersEquityandStockOptionPlansStockbasedCompensationExpenseDetails"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/StockholdersEquityandStockOptionPlansStockbasedCompensationExpenseDetails" xlink:type="extended" id="iec924cfde89a4017845a50c9ca6ffdbb_StockholdersEquityandStockOptionPlansStockbasedCompensationExpenseDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_8048794c-19db-4e06-83ac-1b4483a17a57" 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_b735ac08-1019-4c98-a8c0-57083297d530" 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_8048794c-19db-4e06-83ac-1b4483a17a57" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_b735ac08-1019-4c98-a8c0-57083297d530" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_pubm_ShareBasedPaymentArrangementTaxBenefit_26e70fdd-0031-4988-b976-41eabebae61b" xlink:href="pubm-20210630.xsd#pubm_ShareBasedPaymentArrangementTaxBenefit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_8048794c-19db-4e06-83ac-1b4483a17a57" xlink:to="loc_pubm_ShareBasedPaymentArrangementTaxBenefit_26e70fdd-0031-4988-b976-41eabebae61b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_pubm_ShareBasedPaymentArrangementExpenseNet_bd076c53-5bdc-4444-8bd3-f605bdcd0614" xlink:href="pubm-20210630.xsd#pubm_ShareBasedPaymentArrangementExpenseNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_8048794c-19db-4e06-83ac-1b4483a17a57" xlink:to="loc_pubm_ShareBasedPaymentArrangementExpenseNet_bd076c53-5bdc-4444-8bd3-f605bdcd0614" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_be345397-9e73-484b-999b-cc3aae90045c" 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_8048794c-19db-4e06-83ac-1b4483a17a57" xlink:to="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_be345397-9e73-484b-999b-cc3aae90045c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_fc27ab1e-e361-46ed-9c33-2548f2f305ab" 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_be345397-9e73-484b-999b-cc3aae90045c" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_fc27ab1e-e361-46ed-9c33-2548f2f305ab" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_fc27ab1e-e361-46ed-9c33-2548f2f305ab_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_fc27ab1e-e361-46ed-9c33-2548f2f305ab" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_fc27ab1e-e361-46ed-9c33-2548f2f305ab_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_6f8ee82a-339c-41d4-86ea-572bf37477b4" 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_fc27ab1e-e361-46ed-9c33-2548f2f305ab" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_6f8ee82a-339c-41d4-86ea-572bf37477b4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfSalesMember_180428f3-85bf-4bdb-b1b7-53d83eb1d029" 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_6f8ee82a-339c-41d4-86ea-572bf37477b4" xlink:to="loc_us-gaap_CostOfSalesMember_180428f3-85bf-4bdb-b1b7-53d83eb1d029" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpenseMember_416f64ff-3682-4238-89cf-9da7cdf4c039" 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_6f8ee82a-339c-41d4-86ea-572bf37477b4" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpenseMember_416f64ff-3682-4238-89cf-9da7cdf4c039" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingAndMarketingExpenseMember_521b8abb-102a-49d7-a025-350b1ee5d1ec" 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_6f8ee82a-339c-41d4-86ea-572bf37477b4" xlink:to="loc_us-gaap_SellingAndMarketingExpenseMember_521b8abb-102a-49d7-a025-350b1ee5d1ec" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GeneralAndAdministrativeExpenseMember_1b46eecd-2d68-470f-ab4b-ff1c988fa06f" 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_6f8ee82a-339c-41d4-86ea-572bf37477b4" xlink:to="loc_us-gaap_GeneralAndAdministrativeExpenseMember_1b46eecd-2d68-470f-ab4b-ff1c988fa06f" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://pubmatic.com/role/NetIncomePerShareAttributabletoCommonStockholdersBasicandDilutedNetIncomeperShareDetails" xlink:type="simple" xlink:href="pubm-20210630.xsd#NetIncomePerShareAttributabletoCommonStockholdersBasicandDilutedNetIncomeperShareDetails"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/NetIncomePerShareAttributabletoCommonStockholdersBasicandDilutedNetIncomeperShareDetails" xlink:type="extended" id="ife290aef8fbe44138b4f43e7eb3a7510_NetIncomePerShareAttributabletoCommonStockholdersBasicandDilutedNetIncomeperShareDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareBasicLineItems_1dc65a3c-e416-4e9d-9737-fcf8aa6f9e8c" 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_0aea282d-592b-47d5-8cae-cc35da29847d" 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_1dc65a3c-e416-4e9d-9737-fcf8aa6f9e8c" xlink:to="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersBasicAbstract_0aea282d-592b-47d5-8cae-cc35da29847d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_fa51ed32-8108-4dcd-9e85-1d803e723764" 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_0aea282d-592b-47d5-8cae-cc35da29847d" xlink:to="loc_us-gaap_NetIncomeLoss_fa51ed32-8108-4dcd-9e85-1d803e723764" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UndistributedEarningsLossAllocatedToParticipatingSecuritiesBasic_9379c19f-5e43-4982-936f-949bc2245a75" 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_0aea282d-592b-47d5-8cae-cc35da29847d" xlink:to="loc_us-gaap_UndistributedEarningsLossAllocatedToParticipatingSecuritiesBasic_9379c19f-5e43-4982-936f-949bc2245a75" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_pubm_ReallocationOfNetIncomeLossAttributableToCommonStockholders_6134eb69-25a9-444d-af4c-a7dd2710e414" xlink:href="pubm-20210630.xsd#pubm_ReallocationOfNetIncomeLossAttributableToCommonStockholders"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersBasicAbstract_0aea282d-592b-47d5-8cae-cc35da29847d" xlink:to="loc_pubm_ReallocationOfNetIncomeLossAttributableToCommonStockholders_6134eb69-25a9-444d-af4c-a7dd2710e414" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic_7f834b79-bf94-4ddb-86ce-866dcbbc77b2" 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_0aea282d-592b-47d5-8cae-cc35da29847d" xlink:to="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic_7f834b79-bf94-4ddb-86ce-866dcbbc77b2" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasicAbstract_2b2f47d6-3a35-4d2c-a215-c53d3827a85e" 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_1dc65a3c-e416-4e9d-9737-fcf8aa6f9e8c" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasicAbstract_2b2f47d6-3a35-4d2c-a215-c53d3827a85e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_a7bc6a32-7608-4428-aa1c-224d372cd23e" 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_2b2f47d6-3a35-4d2c-a215-c53d3827a85e" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_a7bc6a32-7608-4428-aa1c-224d372cd23e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareBasic_bf55f645-fd05-4b36-ab66-fe9a71d30fd1" 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_1dc65a3c-e416-4e9d-9737-fcf8aa6f9e8c" xlink:to="loc_us-gaap_EarningsPerShareBasic_bf55f645-fd05-4b36-ab66-fe9a71d30fd1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersDilutedAbstract_d5bf5390-221f-4a50-8829-9c65731e93f4" 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_1dc65a3c-e416-4e9d-9737-fcf8aa6f9e8c" xlink:to="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersDilutedAbstract_d5bf5390-221f-4a50-8829-9c65731e93f4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersDiluted_40ff4de0-856e-4770-9e64-a1484db02ab7" 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_d5bf5390-221f-4a50-8829-9c65731e93f4" xlink:to="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersDiluted_40ff4de0-856e-4770-9e64-a1484db02ab7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_16cbf7f3-1681-4aa6-a0e9-29b5795025ba" 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_1dc65a3c-e416-4e9d-9737-fcf8aa6f9e8c" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_16cbf7f3-1681-4aa6-a0e9-29b5795025ba" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_26133c8f-1316-4280-8d54-4028fbf13fd0" 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_db31b92f-44bf-42c7-9764-51c1ffe541f2" xlink:href="pubm-20210630.xsd#pubm_IncrementalCommonSharesAttributableToDilutiveEffectOfCallOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_16cbf7f3-1681-4aa6-a0e9-29b5795025ba" xlink:to="loc_pubm_IncrementalCommonSharesAttributableToDilutiveEffectOfCallOptions_db31b92f-44bf-42c7-9764-51c1ffe541f2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_pubm_IncrementalCommonSharesAttributableToRestrictedStock_20b4393e-d287-4784-ae14-779b6cfde3c0" xlink:href="pubm-20210630.xsd#pubm_IncrementalCommonSharesAttributableToRestrictedStock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_16cbf7f3-1681-4aa6-a0e9-29b5795025ba" xlink:to="loc_pubm_IncrementalCommonSharesAttributableToRestrictedStock_20b4393e-d287-4784-ae14-779b6cfde3c0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_pubm_IncrementalCommonSharesAttributableToEmployeeStockPurchasePlan_3d0cf1f4-1589-45bb-a1c9-c8439ba86fc3" xlink:href="pubm-20210630.xsd#pubm_IncrementalCommonSharesAttributableToEmployeeStockPurchasePlan"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_16cbf7f3-1681-4aa6-a0e9-29b5795025ba" xlink:to="loc_pubm_IncrementalCommonSharesAttributableToEmployeeStockPurchasePlan_3d0cf1f4-1589-45bb-a1c9-c8439ba86fc3" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_b376e6cf-d47b-483a-b75f-65d9d297bedb" 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_16cbf7f3-1681-4aa6-a0e9-29b5795025ba" xlink:to="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_b376e6cf-d47b-483a-b75f-65d9d297bedb" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareDiluted_6242eaa0-31d9-4896-ab72-670ad08bf5f7" 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_1dc65a3c-e416-4e9d-9737-fcf8aa6f9e8c" xlink:to="loc_us-gaap_EarningsPerShareDiluted_6242eaa0-31d9-4896-ab72-670ad08bf5f7" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEarningsPerShareDilutedByCommonClassTable_bb717233-e682-47e6-872f-58dd98d681b4" 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_1dc65a3c-e416-4e9d-9737-fcf8aa6f9e8c" xlink:to="loc_us-gaap_ScheduleOfEarningsPerShareDilutedByCommonClassTable_bb717233-e682-47e6-872f-58dd98d681b4" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_190e2322-c1ce-4dac-b3d5-0bfd273ba657" 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_bb717233-e682-47e6-872f-58dd98d681b4" xlink:to="loc_us-gaap_StatementClassOfStockAxis_190e2322-c1ce-4dac-b3d5-0bfd273ba657" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_190e2322-c1ce-4dac-b3d5-0bfd273ba657_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_190e2322-c1ce-4dac-b3d5-0bfd273ba657" xlink:to="loc_us-gaap_ClassOfStockDomain_190e2322-c1ce-4dac-b3d5-0bfd273ba657_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_33cbcff5-e582-4b28-8dde-c75aaee1e42c" 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_190e2322-c1ce-4dac-b3d5-0bfd273ba657" xlink:to="loc_us-gaap_ClassOfStockDomain_33cbcff5-e582-4b28-8dde-c75aaee1e42c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_b4fec38b-0893-4d40-86e6-40922d654eef" 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_33cbcff5-e582-4b28-8dde-c75aaee1e42c" xlink:to="loc_us-gaap_CommonClassAMember_b4fec38b-0893-4d40-86e6-40922d654eef" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_fc00cdd9-bbc8-44e4-8cc1-3bb3d9dd440c" 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_33cbcff5-e582-4b28-8dde-c75aaee1e42c" xlink:to="loc_us-gaap_CommonClassBMember_fc00cdd9-bbc8-44e4-8cc1-3bb3d9dd440c" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://pubmatic.com/role/NetIncomePerShareAttributabletoCommonStockholdersScheduleofAntidilutiveSecuritiesExcludedfromComputationofEarningsperShareDetails" xlink:type="simple" xlink:href="pubm-20210630.xsd#NetIncomePerShareAttributabletoCommonStockholdersScheduleofAntidilutiveSecuritiesExcludedfromComputationofEarningsperShareDetails"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/NetIncomePerShareAttributabletoCommonStockholdersScheduleofAntidilutiveSecuritiesExcludedfromComputationofEarningsperShareDetails" xlink:type="extended" id="i63ad34d741ad484aaa5f83c453fe6f60_NetIncomePerShareAttributabletoCommonStockholdersScheduleofAntidilutiveSecuritiesExcludedfromComputationofEarningsperShareDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareDilutedLineItems_fc733861-05b7-4bbf-a719-e00540be11f2" 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_aa0c72ab-44a6-497d-97e7-0c6210894f1c" 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_fc733861-05b7-4bbf-a719-e00540be11f2" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_aa0c72ab-44a6-497d-97e7-0c6210894f1c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEarningsPerShareDilutedByCommonClassTable_e04858cb-70a3-4239-815b-a711ecf350e3" 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_fc733861-05b7-4bbf-a719-e00540be11f2" xlink:to="loc_us-gaap_ScheduleOfEarningsPerShareDilutedByCommonClassTable_e04858cb-70a3-4239-815b-a711ecf350e3" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_5e9c158e-39a6-47e3-9c8c-30deab096989" 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_e04858cb-70a3-4239-815b-a711ecf350e3" xlink:to="loc_us-gaap_StatementClassOfStockAxis_5e9c158e-39a6-47e3-9c8c-30deab096989" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_5e9c158e-39a6-47e3-9c8c-30deab096989_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_5e9c158e-39a6-47e3-9c8c-30deab096989" xlink:to="loc_us-gaap_ClassOfStockDomain_5e9c158e-39a6-47e3-9c8c-30deab096989_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_ab43d2af-e8a1-4a61-8505-4eee3ee718fc" 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_5e9c158e-39a6-47e3-9c8c-30deab096989" xlink:to="loc_us-gaap_ClassOfStockDomain_ab43d2af-e8a1-4a61-8505-4eee3ee718fc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_69161a00-605a-4d9c-8fab-7e8b2ea63864" 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_ab43d2af-e8a1-4a61-8505-4eee3ee718fc" xlink:to="loc_us-gaap_CommonClassAMember_69161a00-605a-4d9c-8fab-7e8b2ea63864" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_5ebaf61a-a6ac-406d-bfd5-aa0c45420813" 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_ab43d2af-e8a1-4a61-8505-4eee3ee718fc" xlink:to="loc_us-gaap_CommonClassBMember_5ebaf61a-a6ac-406d-bfd5-aa0c45420813" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_32a534c3-629b-406f-8c38-e3bb5636f07f" 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_e04858cb-70a3-4239-815b-a711ecf350e3" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_32a534c3-629b-406f-8c38-e3bb5636f07f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_32a534c3-629b-406f-8c38-e3bb5636f07f_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_32a534c3-629b-406f-8c38-e3bb5636f07f" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_32a534c3-629b-406f-8c38-e3bb5636f07f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_eaa67f29-d09b-408f-aa86-2885500bdaeb" 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_32a534c3-629b-406f-8c38-e3bb5636f07f" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_eaa67f29-d09b-408f-aa86-2885500bdaeb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_89cfd2e2-8bbe-4952-801b-2c6ddef6f8a4" 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_eaa67f29-d09b-408f-aa86-2885500bdaeb" xlink:to="loc_us-gaap_EmployeeStockOptionMember_89cfd2e2-8bbe-4952-801b-2c6ddef6f8a4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertiblePreferredStockMember_0d8315d8-e446-47a2-932b-02cca1f6333c" 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_eaa67f29-d09b-408f-aa86-2885500bdaeb" xlink:to="loc_us-gaap_ConvertiblePreferredStockMember_0d8315d8-e446-47a2-932b-02cca1f6333c" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://pubmatic.com/role/IncomeTaxesNarrativeDetails" xlink:type="simple" xlink:href="pubm-20210630.xsd#IncomeTaxesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/IncomeTaxesNarrativeDetails" xlink:type="extended" id="i8777802635074a2eb3f22af2ee1619c7_IncomeTaxesNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxContingencyLineItems_90691961-9832-471c-8c8d-0ca577f615da" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_IncomeTaxContingencyLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_e70a801b-f2c7-4a71-a159-663533a837dd" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_IncomeTaxExpenseBenefit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxContingencyLineItems_90691961-9832-471c-8c8d-0ca577f615da" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_e70a801b-f2c7-4a71-a159-663533a837dd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperations_9512fc8b-f64d-4f90-98f5-f5b9aabbee74" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EffectiveIncomeTaxRateContinuingOperations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxContingencyLineItems_90691961-9832-471c-8c8d-0ca577f615da" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperations_9512fc8b-f64d-4f90-98f5-f5b9aabbee74" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxContingencyTable_ab566046-a041-43fb-9f9c-3b4d2a399cef" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_IncomeTaxContingencyTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_IncomeTaxContingencyLineItems_90691961-9832-471c-8c8d-0ca577f615da" xlink:to="loc_us-gaap_IncomeTaxContingencyTable_ab566046-a041-43fb-9f9c-3b4d2a399cef" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_4139b346-d96a-4642-b09c-bc68115c6578" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_IncomeTaxContingencyTable_ab566046-a041-43fb-9f9c-3b4d2a399cef" xlink:to="loc_srt_RangeAxis_4139b346-d96a-4642-b09c-bc68115c6578" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_4139b346-d96a-4642-b09c-bc68115c6578_default" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_4139b346-d96a-4642-b09c-bc68115c6578" xlink:to="loc_srt_RangeMember_4139b346-d96a-4642-b09c-bc68115c6578_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_fd3853f3-1963-4b4c-a115-8bef43440577" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_4139b346-d96a-4642-b09c-bc68115c6578" xlink:to="loc_srt_RangeMember_fd3853f3-1963-4b4c-a115-8bef43440577" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_cf43403a-2e1f-4d7d-bc3b-2bbd919f0faf" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_fd3853f3-1963-4b4c-a115-8bef43440577" xlink:to="loc_srt_MaximumMember_cf43403a-2e1f-4d7d-bc3b-2bbd919f0faf" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://pubmatic.com/role/SegmentReportingDetails" xlink:type="simple" xlink:href="pubm-20210630.xsd#SegmentReportingDetails"/>
  <link:definitionLink xlink:role="http://pubmatic.com/role/SegmentReportingDetails" xlink:type="extended" id="i25599349ff0a4623927c6b270ee7c5a5_SegmentReportingDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_174f00a7-6822-44ba-af03-04c15969be5c" 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_919e5994-d1f6-40cc-9484-9020ae28aed5" 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_174f00a7-6822-44ba-af03-04c15969be5c" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_919e5994-d1f6-40cc-9484-9020ae28aed5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoncurrentAssets_8eef1a31-3ced-4b05-a876-67b7e0157141" 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_174f00a7-6822-44ba-af03-04c15969be5c" xlink:to="loc_us-gaap_NoncurrentAssets_8eef1a31-3ced-4b05-a876-67b7e0157141" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable_123e62e9-bbdc-42e3-b8ba-e3233b8998da" 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_174f00a7-6822-44ba-af03-04c15969be5c" xlink:to="loc_us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable_123e62e9-bbdc-42e3-b8ba-e3233b8998da" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_c9df826f-6e08-4178-8237-0f821708fbf3" 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_123e62e9-bbdc-42e3-b8ba-e3233b8998da" xlink:to="loc_srt_StatementGeographicalAxis_c9df826f-6e08-4178-8237-0f821708fbf3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_c9df826f-6e08-4178-8237-0f821708fbf3_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_c9df826f-6e08-4178-8237-0f821708fbf3" xlink:to="loc_srt_SegmentGeographicalDomain_c9df826f-6e08-4178-8237-0f821708fbf3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_3cce4293-87fd-4d08-9049-b292d591467d" 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_c9df826f-6e08-4178-8237-0f821708fbf3" xlink:to="loc_srt_SegmentGeographicalDomain_3cce4293-87fd-4d08-9049-b292d591467d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_fd12f8e1-9e07-4b69-8ee2-2482aa9f7137" 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_3cce4293-87fd-4d08-9049-b292d591467d" xlink:to="loc_country_US_fd12f8e1-9e07-4b69-8ee2-2482aa9f7137" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EMEAMember_490d16d5-f0d5-4283-afc2-3e115c46b230" 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_3cce4293-87fd-4d08-9049-b292d591467d" xlink:to="loc_us-gaap_EMEAMember_490d16d5-f0d5-4283-afc2-3e115c46b230" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_AsiaPacificMember_342f0ad8-d018-4903-8327-612151b022da" 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_3cce4293-87fd-4d08-9049-b292d591467d" xlink:to="loc_srt_AsiaPacificMember_342f0ad8-d018-4903-8327-612151b022da" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_pubm_RestOfTheWorldMember_efe5d8cc-ba8b-498d-8c2f-2d0fa1342d64" xlink:href="pubm-20210630.xsd#pubm_RestOfTheWorldMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_3cce4293-87fd-4d08-9049-b292d591467d" xlink:to="loc_pubm_RestOfTheWorldMember_efe5d8cc-ba8b-498d-8c2f-2d0fa1342d64" xlink:type="arc" order="3"/>
  </link:definitionLink>
</link:linkbase>
