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

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2026 Workiva-->
<!--r:019e26c1-fd87-7ee3-80e6-97f74012dbf7,g:c6ed8b33-36b4-46cc-97bf-1271949f0f87-->
<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://www.five9.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITY" xlink:type="simple" xlink:href="fivn-20260630.xsd#CONDENSEDCONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITY"/>
  <link:definitionLink xlink:role="http://www.five9.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITY" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019e26c1-ff33-7528-abec-d0d85ed879f8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019e26c1-ff33-7fd6-aeef-43e18d60a938" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019e26c1-ff33-7528-abec-d0d85ed879f8" xlink:to="loc_us-gaap_StatementTable_019e26c1-ff33-7fd6-aeef-43e18d60a938" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_019e26c1-ff33-79be-b0e5-37cd89fd3899" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019e26c1-ff33-7fd6-aeef-43e18d60a938" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_019e26c1-ff33-79be-b0e5-37cd89fd3899" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019e26c1-ff33-79be-b0e5-37cd89fd3899_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019e26c1-ff33-79be-b0e5-37cd89fd3899" xlink:to="loc_us-gaap_EquityComponentDomain_019e26c1-ff33-79be-b0e5-37cd89fd3899_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019e26c1-ff33-76c4-941f-c8e64d45d12c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019e26c1-ff33-79be-b0e5-37cd89fd3899" xlink:to="loc_us-gaap_EquityComponentDomain_019e26c1-ff33-76c4-941f-c8e64d45d12c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_019e26c1-ff33-7848-be0c-2da40eb72642" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e26c1-ff33-76c4-941f-c8e64d45d12c" xlink:to="loc_us-gaap_CommonStockMember_019e26c1-ff33-7848-be0c-2da40eb72642" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_019e26c1-ff33-7eea-abf6-e72fe843e4f1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e26c1-ff33-76c4-941f-c8e64d45d12c" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_019e26c1-ff33-7eea-abf6-e72fe843e4f1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019e26c1-ff33-75e4-95fa-18e0ca461dc1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e26c1-ff33-76c4-941f-c8e64d45d12c" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019e26c1-ff33-75e4-95fa-18e0ca461dc1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_019e26c1-ff33-7d12-a693-32435eb482d9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e26c1-ff33-76c4-941f-c8e64d45d12c" xlink:to="loc_us-gaap_RetainedEarningsMember_019e26c1-ff33-7d12-a693-32435eb482d9" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e26c1-ff33-724e-9d9f-4137a84af040" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e26c1-ff33-7528-abec-d0d85ed879f8" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e26c1-ff33-724e-9d9f-4137a84af040" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharesIssued_019e26c1-ff33-7913-b1d1-6646768546a9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e26c1-ff33-724e-9d9f-4137a84af040" xlink:to="loc_us-gaap_SharesIssued_019e26c1-ff33-7913-b1d1-6646768546a9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_019e26c1-ff33-7e50-ab41-53c8f0e62fe9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e26c1-ff33-724e-9d9f-4137a84af040" xlink:to="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_019e26c1-ff33-7e50-ab41-53c8f0e62fe9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_019e26c1-ff33-7b8f-b61a-496cebec1945" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e26c1-ff33-724e-9d9f-4137a84af040" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_019e26c1-ff33-7b8f-b61a-496cebec1945" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodValue_019e26c1-ff33-7c51-b3ef-90c1636d5850" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e26c1-ff33-724e-9d9f-4137a84af040" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodValue_019e26c1-ff33-7c51-b3ef-90c1636d5850" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_019e26c1-ff33-7e73-a998-e6c7b5777a3f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e26c1-ff33-724e-9d9f-4137a84af040" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_019e26c1-ff33-7e73-a998-e6c7b5777a3f" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_019e26c1-ff33-7bce-9a44-f8f22ed78981" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e26c1-ff33-724e-9d9f-4137a84af040" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_019e26c1-ff33-7bce-9a44-f8f22ed78981" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation_019e26c1-ff33-7346-8d29-1223f125db64" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e26c1-ff33-724e-9d9f-4137a84af040" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation_019e26c1-ff33-7346-8d29-1223f125db64" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation_019e26c1-ff33-7587-92e2-859810043616" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e26c1-ff33-724e-9d9f-4137a84af040" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation_019e26c1-ff33-7587-92e2-859810043616" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_019e5f07-1c6f-773f-b6a8-7e8d8131c728" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e26c1-ff33-724e-9d9f-4137a84af040" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_019e5f07-1c6f-773f-b6a8-7e8d8131c728" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan_019e5f07-7bb1-7e73-9b9c-a532c80da912" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e26c1-ff33-724e-9d9f-4137a84af040" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan_019e5f07-7bb1-7e73-9b9c-a532c80da912" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_019e26c1-ff33-7635-b655-b267c6dd9d12" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e26c1-ff33-724e-9d9f-4137a84af040" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_019e26c1-ff33-7635-b655-b267c6dd9d12" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_019e26c1-ff33-7a48-95b2-cd19d65039ca" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e26c1-ff33-724e-9d9f-4137a84af040" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_019e26c1-ff33-7a48-95b2-cd19d65039ca" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_019e26c1-ff33-7aa1-b04b-74d601979731" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e26c1-ff33-724e-9d9f-4137a84af040" xlink:to="loc_us-gaap_NetIncomeLoss_019e26c1-ff33-7aa1-b04b-74d601979731" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharesIssued_019e26c1-ff33-7c77-957e-7423d0ec7783" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharesIssued"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_019e26c1-ff33-7939-9fdb-7d922a8df21d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.five9.com/role/DebtTables" xlink:type="simple" xlink:href="fivn-20260630.xsd#DebtTables"/>
  <link:definitionLink xlink:role="http://www.five9.com/role/DebtTables" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019e26c2-0007-7cf8-9e2d-96e69d5f5e2e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019e26c2-0007-76a4-9d4d-720da7bb2559" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e26c2-0007-7cf8-9e2d-96e69d5f5e2e" xlink:to="loc_us-gaap_DebtInstrumentTable_019e26c2-0007-76a4-9d4d-720da7bb2559" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019e26c2-0007-7e8d-9a80-b229029639ba" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e26c2-0007-76a4-9d4d-720da7bb2559" xlink:to="loc_us-gaap_DebtInstrumentAxis_019e26c2-0007-7e8d-9a80-b229029639ba" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e26c2-0007-7e8d-9a80-b229029639ba_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e26c2-0007-7e8d-9a80-b229029639ba" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e26c2-0007-7e8d-9a80-b229029639ba_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e26c2-0007-7069-b3bf-5115dcb4399a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e26c2-0007-7e8d-9a80-b229029639ba" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e26c2-0007-7069-b3bf-5115dcb4399a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_ConvertibleSeniorNotesDue2029Member_019e26c2-0007-704b-b5c8-c3547c248905" xlink:href="fivn-20260630.xsd#fivn_ConvertibleSeniorNotesDue2029Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e26c2-0007-7069-b3bf-5115dcb4399a" xlink:to="loc_fivn_ConvertibleSeniorNotesDue2029Member_019e26c2-0007-704b-b5c8-c3547c248905" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_ConvertibleSeniorNotesDue2025Member_019e26c2-0007-72eb-8df0-0d6eaf47368f" xlink:href="fivn-20260630.xsd#fivn_ConvertibleSeniorNotesDue2025Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e26c2-0007-7069-b3bf-5115dcb4399a" xlink:to="loc_fivn_ConvertibleSeniorNotesDue2025Member_019e26c2-0007-72eb-8df0-0d6eaf47368f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtTableTextBlock_019e26c2-0007-7f97-ba14-7fa96c64475d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConvertibleDebtTableTextBlock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e26c2-0007-7cf8-9e2d-96e69d5f5e2e" xlink:to="loc_us-gaap_ConvertibleDebtTableTextBlock_019e26c2-0007-7f97-ba14-7fa96c64475d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestIncomeAndInterestExpenseDisclosureTableTextBlock_019e26c2-0007-7bd7-810f-df558294cab8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestIncomeAndInterestExpenseDisclosureTableTextBlock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e26c2-0007-7cf8-9e2d-96e69d5f5e2e" xlink:to="loc_us-gaap_InterestIncomeAndInterestExpenseDisclosureTableTextBlock_019e26c2-0007-7bd7-810f-df558294cab8" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.five9.com/role/RevenueNarrativeDetails" xlink:type="simple" xlink:href="fivn-20260630.xsd#RevenueNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.five9.com/role/RevenueNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CapitalizedContractCostLineItems_019e26c1-ff34-71bc-8732-85f9d2565574" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CapitalizedContractCostLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CapitalizedContractCostTable_019e26c1-ff34-7673-9461-2dd9600938a9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CapitalizedContractCostTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_CapitalizedContractCostLineItems_019e26c1-ff34-71bc-8732-85f9d2565574" xlink:to="loc_us-gaap_CapitalizedContractCostTable_019e26c1-ff34-7673-9461-2dd9600938a9" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CapitalizedContractCostAxis_019e26c1-ff34-7c94-a12c-f1d238217d23" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CapitalizedContractCostAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_CapitalizedContractCostTable_019e26c1-ff34-7673-9461-2dd9600938a9" xlink:to="loc_us-gaap_CapitalizedContractCostAxis_019e26c1-ff34-7c94-a12c-f1d238217d23" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CapitalizedContractCostDomain_019e26c1-ff34-7c94-a12c-f1d238217d23_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CapitalizedContractCostDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CapitalizedContractCostAxis_019e26c1-ff34-7c94-a12c-f1d238217d23" xlink:to="loc_us-gaap_CapitalizedContractCostDomain_019e26c1-ff34-7c94-a12c-f1d238217d23_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CapitalizedContractCostDomain_019e26c1-ff34-716d-9598-57fb802755da" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CapitalizedContractCostDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CapitalizedContractCostAxis_019e26c1-ff34-7c94-a12c-f1d238217d23" xlink:to="loc_us-gaap_CapitalizedContractCostDomain_019e26c1-ff34-716d-9598-57fb802755da" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_ContractAcquisitionCostsMember_019e26c1-ff34-7394-bba2-5188c8e99175" xlink:href="fivn-20260630.xsd#fivn_ContractAcquisitionCostsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalizedContractCostDomain_019e26c1-ff34-716d-9598-57fb802755da" xlink:to="loc_fivn_ContractAcquisitionCostsMember_019e26c1-ff34-7394-bba2-5188c8e99175" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis_019e26c1-ff34-785d-ac58-c54ecbbe40cd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_CapitalizedContractCostTable_019e26c1-ff34-7673-9461-2dd9600938a9" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis_019e26c1-ff34-785d-ac58-c54ecbbe40cd" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CapitalizedContractCostAmortizationPeriod_019e26c1-ff34-7ec4-85af-aac0c74fd179" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CapitalizedContractCostAmortizationPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalizedContractCostLineItems_019e26c1-ff34-71bc-8732-85f9d2565574" xlink:to="loc_us-gaap_CapitalizedContractCostAmortizationPeriod_019e26c1-ff34-7ec4-85af-aac0c74fd179" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityRevenueRecognized_019e26c1-ff34-77d8-b534-1b7757059874" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContractWithCustomerLiabilityRevenueRecognized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalizedContractCostLineItems_019e26c1-ff34-71bc-8732-85f9d2565574" xlink:to="loc_us-gaap_ContractWithCustomerLiabilityRevenueRecognized_019e26c1-ff34-77d8-b534-1b7757059874" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligation_019e26c1-ff34-7c92-a3ea-e565ef89e197" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueRemainingPerformanceObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalizedContractCostLineItems_019e26c1-ff34-71bc-8732-85f9d2565574" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligation_019e26c1-ff34-7c92-a3ea-e565ef89e197" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationPercentage_019e26c1-ff34-782f-b7fe-f3e6c45f67df" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueRemainingPerformanceObligationPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalizedContractCostLineItems_019e26c1-ff34-71bc-8732-85f9d2565574" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationPercentage_019e26c1-ff34-782f-b7fe-f3e6c45f67df" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_019e26c1-ff34-7722-8ffa-a9aff12bc8f3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalizedContractCostLineItems_019e26c1-ff34-71bc-8732-85f9d2565574" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_019e26c1-ff34-7722-8ffa-a9aff12bc8f3" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenuePerformanceObligationDescriptionOfTiming_019e26c1-ff34-76ad-beae-53709035df73" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenuePerformanceObligationDescriptionOfTiming"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalizedContractCostLineItems_019e26c1-ff34-71bc-8732-85f9d2565574" xlink:to="loc_us-gaap_RevenuePerformanceObligationDescriptionOfTiming_019e26c1-ff34-76ad-beae-53709035df73" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.five9.com/role/RevenueNarrativeDetails_1" xlink:type="simple" xlink:href="fivn-20260630.xsd#RevenueNarrativeDetails_1"/>
  <link:definitionLink xlink:role="http://www.five9.com/role/RevenueNarrativeDetails_1" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CapitalizedContractCostLineItems_019e26c1-ff34-71bc-8732-85f9d2565574" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CapitalizedContractCostLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CapitalizedContractCostTable_019e26c1-ff34-7673-9461-2dd9600938a9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CapitalizedContractCostTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_CapitalizedContractCostLineItems_019e26c1-ff34-71bc-8732-85f9d2565574" xlink:to="loc_us-gaap_CapitalizedContractCostTable_019e26c1-ff34-7673-9461-2dd9600938a9" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CapitalizedContractCostAxis_019e26c1-ff34-7c94-a12c-f1d238217d23" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CapitalizedContractCostAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_CapitalizedContractCostTable_019e26c1-ff34-7673-9461-2dd9600938a9" xlink:to="loc_us-gaap_CapitalizedContractCostAxis_019e26c1-ff34-7c94-a12c-f1d238217d23" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CapitalizedContractCostDomain_019e26c1-ff34-7c94-a12c-f1d238217d23_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CapitalizedContractCostDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CapitalizedContractCostAxis_019e26c1-ff34-7c94-a12c-f1d238217d23" xlink:to="loc_us-gaap_CapitalizedContractCostDomain_019e26c1-ff34-7c94-a12c-f1d238217d23_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CapitalizedContractCostDomain_019e26c1-ff34-716d-9598-57fb802755da" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CapitalizedContractCostDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CapitalizedContractCostAxis_019e26c1-ff34-7c94-a12c-f1d238217d23" xlink:to="loc_us-gaap_CapitalizedContractCostDomain_019e26c1-ff34-716d-9598-57fb802755da" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_ContractAcquisitionCostsMember_019e26c1-ff34-7394-bba2-5188c8e99175" xlink:href="fivn-20260630.xsd#fivn_ContractAcquisitionCostsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalizedContractCostDomain_019e26c1-ff34-716d-9598-57fb802755da" xlink:to="loc_fivn_ContractAcquisitionCostsMember_019e26c1-ff34-7394-bba2-5188c8e99175" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CapitalizedContractCostAmortizationPeriod_019e26c1-ff34-7ec4-85af-aac0c74fd179" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CapitalizedContractCostAmortizationPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalizedContractCostLineItems_019e26c1-ff34-71bc-8732-85f9d2565574" xlink:to="loc_us-gaap_CapitalizedContractCostAmortizationPeriod_019e26c1-ff34-7ec4-85af-aac0c74fd179" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityRevenueRecognized_019e26c1-ff34-77d8-b534-1b7757059874" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ContractWithCustomerLiabilityRevenueRecognized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalizedContractCostLineItems_019e26c1-ff34-71bc-8732-85f9d2565574" xlink:to="loc_us-gaap_ContractWithCustomerLiabilityRevenueRecognized_019e26c1-ff34-77d8-b534-1b7757059874" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligation_019e26c1-ff34-7c92-a3ea-e565ef89e197" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueRemainingPerformanceObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalizedContractCostLineItems_019e26c1-ff34-71bc-8732-85f9d2565574" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligation_019e26c1-ff34-7c92-a3ea-e565ef89e197" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationPercentage_019e26c1-ff34-782f-b7fe-f3e6c45f67df" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueRemainingPerformanceObligationPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalizedContractCostLineItems_019e26c1-ff34-71bc-8732-85f9d2565574" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationPercentage_019e26c1-ff34-782f-b7fe-f3e6c45f67df" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_019e26c1-ff34-7722-8ffa-a9aff12bc8f3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalizedContractCostLineItems_019e26c1-ff34-71bc-8732-85f9d2565574" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_019e26c1-ff34-7722-8ffa-a9aff12bc8f3" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenuePerformanceObligationDescriptionOfTiming_019e26c1-ff34-76ad-beae-53709035df73" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenuePerformanceObligationDescriptionOfTiming"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalizedContractCostLineItems_019e26c1-ff34-71bc-8732-85f9d2565574" xlink:to="loc_us-gaap_RevenuePerformanceObligationDescriptionOfTiming_019e26c1-ff34-76ad-beae-53709035df73" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.five9.com/role/InvestmentsandFairValueMeasurementsScheduleofMarketableInvestmentsDetails" xlink:type="simple" xlink:href="fivn-20260630.xsd#InvestmentsandFairValueMeasurementsScheduleofMarketableInvestmentsDetails"/>
  <link:definitionLink xlink:role="http://www.five9.com/role/InvestmentsandFairValueMeasurementsScheduleofMarketableInvestmentsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019e26c1-ff34-7e97-a205-fbae64ff64cb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_019e26c1-ff34-79d5-958f-770a66213c17" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019e26c1-ff34-7e97-a205-fbae64ff64cb" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_019e26c1-ff34-79d5-958f-770a66213c17" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019e26c1-ff34-7ff8-8c21-e3d2f8ecefc7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_019e26c1-ff34-79d5-958f-770a66213c17" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019e26c1-ff34-7ff8-8c21-e3d2f8ecefc7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e26c1-ff34-7ff8-8c21-e3d2f8ecefc7_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019e26c1-ff34-7ff8-8c21-e3d2f8ecefc7" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e26c1-ff34-7ff8-8c21-e3d2f8ecefc7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e26c1-ff34-7a0b-8952-daa76913c43c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019e26c1-ff34-7ff8-8c21-e3d2f8ecefc7" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e26c1-ff34-7a0b-8952-daa76913c43c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CertificatesOfDepositMember_019e26c1-ff34-770c-be73-07beced86104" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CertificatesOfDepositMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e26c1-ff34-7a0b-8952-daa76913c43c" xlink:to="loc_us-gaap_CertificatesOfDepositMember_019e26c1-ff34-770c-be73-07beced86104" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USTreasurySecuritiesMember_019e26c1-ff34-7ba6-ab58-6c2faed123e7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_USTreasurySecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e26c1-ff34-7a0b-8952-daa76913c43c" xlink:to="loc_us-gaap_USTreasurySecuritiesMember_019e26c1-ff34-7ba6-ab58-6c2faed123e7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USGovernmentAgenciesDebtSecuritiesMember_019e26c1-ff34-7504-97b0-050301f311ba" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_USGovernmentAgenciesDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e26c1-ff34-7a0b-8952-daa76913c43c" xlink:to="loc_us-gaap_USGovernmentAgenciesDebtSecuritiesMember_019e26c1-ff34-7504-97b0-050301f311ba" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialPaperMember_019e26c1-ff34-7454-b073-a48c2ddc49dd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommercialPaperMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e26c1-ff34-7a0b-8952-daa76913c43c" xlink:to="loc_us-gaap_CommercialPaperMember_019e26c1-ff34-7454-b073-a48c2ddc49dd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MunicipalBondsMember_019e26c1-ff34-775e-a801-31d4fa275da6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MunicipalBondsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e26c1-ff34-7a0b-8952-daa76913c43c" xlink:to="loc_us-gaap_MunicipalBondsMember_019e26c1-ff34-775e-a801-31d4fa275da6" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateBondSecuritiesMember_019e26c1-ff34-716f-bf75-c5b91a4bd179" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CorporateBondSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e26c1-ff34-7a0b-8952-daa76913c43c" xlink:to="loc_us-gaap_CorporateBondSecuritiesMember_019e26c1-ff34-716f-bf75-c5b91a4bd179" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis_019e26c1-ff34-7a4f-98a8-8778cf19dff7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019e26c1-ff34-7e97-a205-fbae64ff64cb" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis_019e26c1-ff34-7a4f-98a8-8778cf19dff7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_019e26c1-ff34-7475-80a3-03b4bd8964ca" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019e26c1-ff34-7e97-a205-fbae64ff64cb" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_019e26c1-ff34-7475-80a3-03b4bd8964ca" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_019e26c1-ff34-7956-973e-800f02a7b663" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019e26c1-ff34-7e97-a205-fbae64ff64cb" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_019e26c1-ff34-7956-973e-800f02a7b663" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_019e26c1-ff34-7bf4-9ea8-846ce58e7682" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019e26c1-ff34-7e97-a205-fbae64ff64cb" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_019e26c1-ff34-7bf4-9ea8-846ce58e7682" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.five9.com/role/InvestmentsandFairValueMeasurementsScheduleofGrossUnrealizedLossesandFairValueofMarketableInvestmentsDetails" xlink:type="simple" xlink:href="fivn-20260630.xsd#InvestmentsandFairValueMeasurementsScheduleofGrossUnrealizedLossesandFairValueofMarketableInvestmentsDetails"/>
  <link:definitionLink xlink:role="http://www.five9.com/role/InvestmentsandFairValueMeasurementsScheduleofGrossUnrealizedLossesandFairValueofMarketableInvestmentsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019e26c1-ff34-7a1a-bea8-a2ad2bb3cea2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_019e26c1-ff34-7e10-b639-c6776b97c8b8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019e26c1-ff34-7a1a-bea8-a2ad2bb3cea2" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_019e26c1-ff34-7e10-b639-c6776b97c8b8" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019e26c1-ff34-77a5-8e55-805cb5273dda" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_019e26c1-ff34-7e10-b639-c6776b97c8b8" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019e26c1-ff34-77a5-8e55-805cb5273dda" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e26c1-ff34-77a5-8e55-805cb5273dda_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019e26c1-ff34-77a5-8e55-805cb5273dda" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e26c1-ff34-77a5-8e55-805cb5273dda_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e26c1-ff34-773e-b2b8-7de3d937e831" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019e26c1-ff34-77a5-8e55-805cb5273dda" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e26c1-ff34-773e-b2b8-7de3d937e831" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CertificatesOfDepositMember_019e26c1-ff34-7721-a8a2-2f5230fdd711" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CertificatesOfDepositMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e26c1-ff34-773e-b2b8-7de3d937e831" xlink:to="loc_us-gaap_CertificatesOfDepositMember_019e26c1-ff34-7721-a8a2-2f5230fdd711" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USTreasurySecuritiesMember_019e26c1-ff34-7803-b0d8-4d15a18f2692" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_USTreasurySecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e26c1-ff34-773e-b2b8-7de3d937e831" xlink:to="loc_us-gaap_USTreasurySecuritiesMember_019e26c1-ff34-7803-b0d8-4d15a18f2692" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USGovernmentAgenciesDebtSecuritiesMember_019e26c1-ff34-7825-bf67-6d4c21b2ae56" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_USGovernmentAgenciesDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e26c1-ff34-773e-b2b8-7de3d937e831" xlink:to="loc_us-gaap_USGovernmentAgenciesDebtSecuritiesMember_019e26c1-ff34-7825-bf67-6d4c21b2ae56" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialPaperMember_019e26c1-ff34-7f5b-8ec6-a0d8d29986c5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommercialPaperMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e26c1-ff34-773e-b2b8-7de3d937e831" xlink:to="loc_us-gaap_CommercialPaperMember_019e26c1-ff34-7f5b-8ec6-a0d8d29986c5" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MunicipalBondsMember_019f65b7-7946-75ac-809a-cab658fd4579" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MunicipalBondsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e26c1-ff34-773e-b2b8-7de3d937e831" xlink:to="loc_us-gaap_MunicipalBondsMember_019f65b7-7946-75ac-809a-cab658fd4579" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateBondSecuritiesMember_019e26c1-ff34-7740-a9b1-62dc1e4551f9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CorporateBondSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e26c1-ff34-773e-b2b8-7de3d937e831" xlink:to="loc_us-gaap_CorporateBondSecuritiesMember_019e26c1-ff34-7740-a9b1-62dc1e4551f9" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss_019e26c1-ff34-7bec-aa93-18f5e255c496" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019e26c1-ff34-7a1a-bea8-a2ad2bb3cea2" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss_019e26c1-ff34-7bec-aa93-18f5e255c496" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss_019e26c1-ff34-7809-90ef-62870566b36c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019e26c1-ff34-7a1a-bea8-a2ad2bb3cea2" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss_019e26c1-ff34-7809-90ef-62870566b36c" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.five9.com/role/InvestmentsandFairValueMeasurementsScheduleofAssetsCarriedatFairValueDetails" xlink:type="simple" xlink:href="fivn-20260630.xsd#InvestmentsandFairValueMeasurementsScheduleofAssetsCarriedatFairValueDetails"/>
  <link:definitionLink xlink:role="http://www.five9.com/role/InvestmentsandFairValueMeasurementsScheduleofAssetsCarriedatFairValueDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019e26c1-ff34-7bbf-904e-408504334738" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019e26c1-ff34-798d-808f-664c82586ec8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019e26c1-ff34-7bbf-904e-408504334738" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019e26c1-ff34-798d-808f-664c82586ec8" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAxis_019e26c1-ff34-735c-b050-4dfa0e25557a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashAndCashEquivalentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019e26c1-ff34-798d-808f-664c82586ec8" xlink:to="loc_us-gaap_CashAndCashEquivalentsAxis_019e26c1-ff34-735c-b050-4dfa0e25557a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019e26c1-ff34-735c-b050-4dfa0e25557a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CashAndCashEquivalentsAxis_019e26c1-ff34-735c-b050-4dfa0e25557a" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019e26c1-ff34-735c-b050-4dfa0e25557a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019e26c1-ff34-7284-9239-75fe905303ff" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CashAndCashEquivalentsAxis_019e26c1-ff34-735c-b050-4dfa0e25557a" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019e26c1-ff34-7284-9239-75fe905303ff" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MoneyMarketFundsMember_019e26c1-ff34-7558-a7ca-4bc2eebb2ca1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MoneyMarketFundsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019e26c1-ff34-7284-9239-75fe905303ff" xlink:to="loc_us-gaap_MoneyMarketFundsMember_019e26c1-ff34-7558-a7ca-4bc2eebb2ca1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CertificatesOfDepositMember_019e26c1-ff34-72a5-a8c1-804b72b1d987" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CertificatesOfDepositMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019e26c1-ff34-7284-9239-75fe905303ff" xlink:to="loc_us-gaap_CertificatesOfDepositMember_019e26c1-ff34-72a5-a8c1-804b72b1d987" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USTreasurySecuritiesMember_019e26c1-ff34-79e7-b3c2-58a416edbbe9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_USTreasurySecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019e26c1-ff34-7284-9239-75fe905303ff" xlink:to="loc_us-gaap_USTreasurySecuritiesMember_019e26c1-ff34-79e7-b3c2-58a416edbbe9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e26c1-ff34-764b-b5fe-b3b2933d993b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019e26c1-ff34-798d-808f-664c82586ec8" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e26c1-ff34-764b-b5fe-b3b2933d993b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e26c1-ff34-764b-b5fe-b3b2933d993b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e26c1-ff34-764b-b5fe-b3b2933d993b" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e26c1-ff34-764b-b5fe-b3b2933d993b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e26c1-ff34-7bfa-b566-fe60368d0d00" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e26c1-ff34-764b-b5fe-b3b2933d993b" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e26c1-ff34-7bfa-b566-fe60368d0d00" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_019e26c1-ff34-79b3-9bd9-38b050d63dfe" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e26c1-ff34-7bfa-b566-fe60368d0d00" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_019e26c1-ff34-79b3-9bd9-38b050d63dfe" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_019e26c1-ff34-7b38-b011-ce3a97c77707" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e26c1-ff34-7bfa-b566-fe60368d0d00" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_019e26c1-ff34-7b38-b011-ce3a97c77707" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_019e26c1-ff34-7746-881e-6c34dbff285a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e26c1-ff34-7bfa-b566-fe60368d0d00" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_019e26c1-ff34-7746-881e-6c34dbff285a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019e26c1-ff34-7494-8201-a7469e6ccda9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019e26c1-ff34-798d-808f-664c82586ec8" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019e26c1-ff34-7494-8201-a7469e6ccda9" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e26c1-ff34-7494-8201-a7469e6ccda9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019e26c1-ff34-7494-8201-a7469e6ccda9" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e26c1-ff34-7494-8201-a7469e6ccda9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e26c1-ff34-72fd-9598-d7ceefd1c716" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019e26c1-ff34-7494-8201-a7469e6ccda9" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e26c1-ff34-72fd-9598-d7ceefd1c716" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CertificatesOfDepositMember_019e26c1-ff34-765e-b508-147f49ddde45" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CertificatesOfDepositMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e26c1-ff34-72fd-9598-d7ceefd1c716" xlink:to="loc_us-gaap_CertificatesOfDepositMember_019e26c1-ff34-765e-b508-147f49ddde45" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USTreasurySecuritiesMember_019e26c1-ff34-750f-938d-cec773d24064" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_USTreasurySecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e26c1-ff34-72fd-9598-d7ceefd1c716" xlink:to="loc_us-gaap_USTreasurySecuritiesMember_019e26c1-ff34-750f-938d-cec773d24064" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_USGovernmentAgenciesAndUSGovernmentSponsoredEnterprisesDebtSecuritiesMember_019e26c1-ff34-7a03-bc05-47181ac83584" xlink:href="fivn-20260630.xsd#fivn_USGovernmentAgenciesAndUSGovernmentSponsoredEnterprisesDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e26c1-ff34-72fd-9598-d7ceefd1c716" xlink:to="loc_fivn_USGovernmentAgenciesAndUSGovernmentSponsoredEnterprisesDebtSecuritiesMember_019e26c1-ff34-7a03-bc05-47181ac83584" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialPaperMember_019e26c1-ff34-7c7e-98c2-8e35f8085e09" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommercialPaperMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e26c1-ff34-72fd-9598-d7ceefd1c716" xlink:to="loc_us-gaap_CommercialPaperMember_019e26c1-ff34-7c7e-98c2-8e35f8085e09" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MunicipalBondsMember_019e26c1-ff34-7de6-8cda-fb3b46651969" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MunicipalBondsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e26c1-ff34-72fd-9598-d7ceefd1c716" xlink:to="loc_us-gaap_MunicipalBondsMember_019e26c1-ff34-7de6-8cda-fb3b46651969" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateBondSecuritiesMember_019e26c1-ff34-7acc-8a3c-23d65b7570df" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CorporateBondSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e26c1-ff34-72fd-9598-d7ceefd1c716" xlink:to="loc_us-gaap_CorporateBondSecuritiesMember_019e26c1-ff34-7acc-8a3c-23d65b7570df" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsAbstract_019e26c1-ff34-7680-a160-94f121314a94" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssetsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019e26c1-ff34-7bbf-904e-408504334738" xlink:to="loc_us-gaap_AssetsAbstract_019e26c1-ff34-7680-a160-94f121314a94" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_019e26c1-ff34-7652-b314-c06440ea902f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashAndCashEquivalentsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019e26c1-ff34-7680-a160-94f121314a94" xlink:to="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_019e26c1-ff34-7652-b314-c06440ea902f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_019e26c1-ff34-707a-841a-6ec93158c8a7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019e26c1-ff34-7680-a160-94f121314a94" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_019e26c1-ff34-707a-841a-6ec93158c8a7" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.five9.com/role/InvestmentsandFairValueMeasurementsNarrativeDetails" xlink:type="simple" xlink:href="fivn-20260630.xsd#InvestmentsandFairValueMeasurementsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.five9.com/role/InvestmentsandFairValueMeasurementsNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019e26c1-ff34-728c-96a8-10e4079d1caa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019e26c1-ff34-75e6-a83c-ea77006e4c55" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e26c1-ff34-728c-96a8-10e4079d1caa" xlink:to="loc_us-gaap_DebtInstrumentTable_019e26c1-ff34-75e6-a83c-ea77006e4c55" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019e26c1-ff34-7d46-bc56-0fdace390cc9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e26c1-ff34-75e6-a83c-ea77006e4c55" xlink:to="loc_us-gaap_DebtInstrumentAxis_019e26c1-ff34-7d46-bc56-0fdace390cc9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e26c1-ff34-7d46-bc56-0fdace390cc9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e26c1-ff34-7d46-bc56-0fdace390cc9" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e26c1-ff34-7d46-bc56-0fdace390cc9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e26c1-ff34-7a12-88a2-c62f6f59e841" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e26c1-ff34-7d46-bc56-0fdace390cc9" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e26c1-ff34-7a12-88a2-c62f6f59e841" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_ConvertibleSeniorNotesDue2029Member_019e26c1-ff34-7cb7-82b6-9018e92396bc" xlink:href="fivn-20260630.xsd#fivn_ConvertibleSeniorNotesDue2029Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e26c1-ff34-7a12-88a2-c62f6f59e841" xlink:to="loc_fivn_ConvertibleSeniorNotesDue2029Member_019e26c1-ff34-7cb7-82b6-9018e92396bc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_ConvertibleSeniorNotesDue2025Member_019e26c1-ff34-78bb-9c2e-a2c78d46c7ba" xlink:href="fivn-20260630.xsd#fivn_ConvertibleSeniorNotesDue2025Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e26c1-ff34-7a12-88a2-c62f6f59e841" xlink:to="loc_fivn_ConvertibleSeniorNotesDue2025Member_019e26c1-ff34-78bb-9c2e-a2c78d46c7ba" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019e26c1-ff34-7bc6-819a-73b95f7e7981" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e26c1-ff34-75e6-a83c-ea77006e4c55" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019e26c1-ff34-7bc6-819a-73b95f7e7981" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e26c1-ff34-7bc6-819a-73b95f7e7981_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e26c1-ff34-7bc6-819a-73b95f7e7981" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e26c1-ff34-7bc6-819a-73b95f7e7981_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e26c1-ff34-7950-a5cc-c7aae99af5bd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e26c1-ff34-7bc6-819a-73b95f7e7981" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e26c1-ff34-7950-a5cc-c7aae99af5bd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtMember_019e26c1-ff34-7eae-af63-1549e0960913" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConvertibleDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019e26c1-ff34-7950-a5cc-c7aae99af5bd" xlink:to="loc_us-gaap_ConvertibleDebtMember_019e26c1-ff34-7eae-af63-1549e0960913" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_019e26c1-ff34-704d-b1e1-dcfae3a6b9b0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsidiarySaleOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e26c1-ff34-75e6-a83c-ea77006e4c55" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_019e26c1-ff34-704d-b1e1-dcfae3a6b9b0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019e26c1-ff34-704d-b1e1-dcfae3a6b9b0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019e26c1-ff34-704d-b1e1-dcfae3a6b9b0" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019e26c1-ff34-704d-b1e1-dcfae3a6b9b0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019e26c1-ff34-7872-ad9f-0d5bac34b659" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019e26c1-ff34-704d-b1e1-dcfae3a6b9b0" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019e26c1-ff34-7872-ad9f-0d5bac34b659" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrivatePlacementMember_019e26c1-ff34-7046-8132-52b321e12fc9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PrivatePlacementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019e26c1-ff34-7872-ad9f-0d5bac34b659" xlink:to="loc_us-gaap_PrivatePlacementMember_019e26c1-ff34-7046-8132-52b321e12fc9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e26c1-ff34-7a8b-b254-b3828f8d186d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e26c1-ff34-75e6-a83c-ea77006e4c55" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e26c1-ff34-7a8b-b254-b3828f8d186d" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e26c1-ff34-7a8b-b254-b3828f8d186d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e26c1-ff34-7a8b-b254-b3828f8d186d" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e26c1-ff34-7a8b-b254-b3828f8d186d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e26c1-ff34-7462-b484-8ec8c7a9613d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e26c1-ff34-7a8b-b254-b3828f8d186d" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e26c1-ff34-7462-b484-8ec8c7a9613d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_019e26c1-ff34-7833-9092-699572bfd041" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e26c1-ff34-7462-b484-8ec8c7a9613d" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_019e26c1-ff34-7833-9092-699572bfd041" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementFrequencyAxis_019f6642-d6cd-7881-912a-3c7aaad84dd0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByMeasurementFrequencyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e26c1-ff34-75e6-a83c-ea77006e4c55" xlink:to="loc_us-gaap_FairValueByMeasurementFrequencyAxis_019f6642-d6cd-7881-912a-3c7aaad84dd0" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_019f6642-d6cd-7881-912a-3c7aaad84dd0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_019f6642-d6cd-7881-912a-3c7aaad84dd0" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_019f6642-d6cd-7881-912a-3c7aaad84dd0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_019f6642-d6cd-7b51-aafa-2873305356d2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_019f6642-d6cd-7881-912a-3c7aaad84dd0" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_019f6642-d6cd-7b51-aafa-2873305356d2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsNonrecurringMember_019f6643-73a1-72c3-b617-02fe61af659c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsNonrecurringMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementFrequencyDomain_019f6642-d6cd-7b51-aafa-2873305356d2" xlink:to="loc_us-gaap_FairValueMeasurementsNonrecurringMember_019f6643-73a1-72c3-b617-02fe61af659c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_019e26c1-ff34-71ca-ad1c-4599fd651e64" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e26c1-ff34-728c-96a8-10e4079d1caa" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_019e26c1-ff34-71ca-ad1c-4599fd651e64" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019e26c1-ff34-7c1f-bad8-c3f4cebf2fd0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e26c1-ff34-728c-96a8-10e4079d1caa" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019e26c1-ff34-7c1f-bad8-c3f4cebf2fd0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ExtinguishmentOfDebtAmount_019e26c1-ff34-74eb-af10-c02de1afe9ee" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ExtinguishmentOfDebtAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e26c1-ff34-728c-96a8-10e4079d1caa" xlink:to="loc_us-gaap_ExtinguishmentOfDebtAmount_019e26c1-ff34-74eb-af10-c02de1afe9ee" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtFairValue_019e26c1-ff34-772e-b54b-ae0933073909" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebtFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e26c1-ff34-728c-96a8-10e4079d1caa" xlink:to="loc_us-gaap_LongTermDebtFairValue_019e26c1-ff34-772e-b54b-ae0933073909" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueAmount_019e26c1-ff34-747f-aa0c-ce9958d75629" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e26c1-ff34-728c-96a8-10e4079d1caa" xlink:to="loc_us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueAmount_019e26c1-ff34-747f-aa0c-ce9958d75629" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueImpairmentLossAnnualAmount_019e26c1-ff34-784c-9124-44ea2ebcaa36" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueImpairmentLossAnnualAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e26c1-ff34-728c-96a8-10e4079d1caa" xlink:to="loc_us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueImpairmentLossAnnualAmount_019e26c1-ff34-784c-9124-44ea2ebcaa36" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueNetAssetLiability_019e26c1-ff34-78d3-b04b-096b6e5763d2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueNetAssetLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e26c1-ff34-728c-96a8-10e4079d1caa" xlink:to="loc_us-gaap_FairValueNetAssetLiability_019e26c1-ff34-78d3-b04b-096b6e5763d2" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.five9.com/role/FinancialStatementComponentsScheduleofCashandCashEquivalentsDetails" xlink:type="simple" xlink:href="fivn-20260630.xsd#FinancialStatementComponentsScheduleofCashandCashEquivalentsDetails"/>
  <link:definitionLink xlink:role="http://www.five9.com/role/FinancialStatementComponentsScheduleofCashandCashEquivalentsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsLineItems_019e26c1-ff34-788f-b4b2-2841dd5ca0cb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashAndCashEquivalentsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfCashAndCashEquivalentsTable_019e26c1-ff34-75b8-96cf-702518a4ba4b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfCashAndCashEquivalentsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_019e26c1-ff34-788f-b4b2-2841dd5ca0cb" xlink:to="loc_us-gaap_ScheduleOfCashAndCashEquivalentsTable_019e26c1-ff34-75b8-96cf-702518a4ba4b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAxis_019e26c1-ff34-7b35-b3d7-8e6dcc2f36cd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashAndCashEquivalentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCashAndCashEquivalentsTable_019e26c1-ff34-75b8-96cf-702518a4ba4b" xlink:to="loc_us-gaap_CashAndCashEquivalentsAxis_019e26c1-ff34-7b35-b3d7-8e6dcc2f36cd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019e26c1-ff34-7b35-b3d7-8e6dcc2f36cd_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CashAndCashEquivalentsAxis_019e26c1-ff34-7b35-b3d7-8e6dcc2f36cd" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019e26c1-ff34-7b35-b3d7-8e6dcc2f36cd_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019e26c1-ff34-7b71-93d5-a2e21f938302" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CashAndCashEquivalentsAxis_019e26c1-ff34-7b35-b3d7-8e6dcc2f36cd" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019e26c1-ff34-7b71-93d5-a2e21f938302" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashMember_019e26c1-ff34-7bcd-a8a0-fe76184bf9ec" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019e26c1-ff34-7b71-93d5-a2e21f938302" xlink:to="loc_us-gaap_CashMember_019e26c1-ff34-7bcd-a8a0-fe76184bf9ec" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MoneyMarketFundsMember_019e26c1-ff34-7470-b9ad-c9fdb8faaf16" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MoneyMarketFundsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019e26c1-ff34-7b71-93d5-a2e21f938302" xlink:to="loc_us-gaap_MoneyMarketFundsMember_019e26c1-ff34-7470-b9ad-c9fdb8faaf16" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CertificatesOfDepositMember_019e26c1-ff34-743b-8b32-90a7c4db2079" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CertificatesOfDepositMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019e26c1-ff34-7b71-93d5-a2e21f938302" xlink:to="loc_us-gaap_CertificatesOfDepositMember_019e26c1-ff34-743b-8b32-90a7c4db2079" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USTreasurySecuritiesMember_019e26c1-ff34-7ba1-be18-7bbc1d29ef1d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_USTreasurySecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019e26c1-ff34-7b71-93d5-a2e21f938302" xlink:to="loc_us-gaap_USTreasurySecuritiesMember_019e26c1-ff34-7ba1-be18-7bbc1d29ef1d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_019e26c1-ff34-739a-9793-c3873dfef035" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_019e26c1-ff34-788f-b4b2-2841dd5ca0cb" xlink:to="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_019e26c1-ff34-739a-9793-c3873dfef035" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.five9.com/role/FinancialStatementComponentsScheduleofAccountsReceivableDetails" xlink:type="simple" xlink:href="fivn-20260630.xsd#FinancialStatementComponentsScheduleofAccountsReceivableDetails"/>
  <link:definitionLink xlink:role="http://www.five9.com/role/FinancialStatementComponentsScheduleofAccountsReceivableDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsNotesAndLoansReceivableLineItems_019e26c1-ff34-7cbe-ad54-60eee77b97d3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccountsNotesAndLoansReceivableLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAccountsNotesLoansAndFinancingReceivableTable_019e26c1-ff34-7ebd-a9d9-82f1ccab248e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfAccountsNotesLoansAndFinancingReceivableTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AccountsNotesAndLoansReceivableLineItems_019e26c1-ff34-7cbe-ad54-60eee77b97d3" xlink:to="loc_us-gaap_ScheduleOfAccountsNotesLoansAndFinancingReceivableTable_019e26c1-ff34-7ebd-a9d9-82f1ccab248e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsNotesLoansAndFinancingReceivablesByBillingStatusTypeAxis_019e26c1-ff34-746e-8a57-bb35a489cb70" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccountsNotesLoansAndFinancingReceivablesByBillingStatusTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAccountsNotesLoansAndFinancingReceivableTable_019e26c1-ff34-7ebd-a9d9-82f1ccab248e" xlink:to="loc_us-gaap_AccountsNotesLoansAndFinancingReceivablesByBillingStatusTypeAxis_019e26c1-ff34-746e-8a57-bb35a489cb70" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReceivablesBillingStatusDomain_019e26c1-ff34-746e-8a57-bb35a489cb70_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReceivablesBillingStatusDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AccountsNotesLoansAndFinancingReceivablesByBillingStatusTypeAxis_019e26c1-ff34-746e-8a57-bb35a489cb70" xlink:to="loc_us-gaap_ReceivablesBillingStatusDomain_019e26c1-ff34-746e-8a57-bb35a489cb70_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReceivablesBillingStatusDomain_019e26c1-ff34-7556-9f42-7678e6d7d3b4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ReceivablesBillingStatusDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AccountsNotesLoansAndFinancingReceivablesByBillingStatusTypeAxis_019e26c1-ff34-746e-8a57-bb35a489cb70" xlink:to="loc_us-gaap_ReceivablesBillingStatusDomain_019e26c1-ff34-7556-9f42-7678e6d7d3b4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BilledRevenuesMember_019e26c1-ff34-7329-bc15-3f2acf5a39d3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BilledRevenuesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReceivablesBillingStatusDomain_019e26c1-ff34-7556-9f42-7678e6d7d3b4" xlink:to="loc_us-gaap_BilledRevenuesMember_019e26c1-ff34-7329-bc15-3f2acf5a39d3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnbilledRevenuesMember_019e26c1-ff34-7a18-87d9-f117e81f0396" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_UnbilledRevenuesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReceivablesBillingStatusDomain_019e26c1-ff34-7556-9f42-7678e6d7d3b4" xlink:to="loc_us-gaap_UnbilledRevenuesMember_019e26c1-ff34-7a18-87d9-f117e81f0396" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsReceivableGrossCurrent_019e26c1-ff34-707f-b5a0-a0aa5b1d6a68" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccountsReceivableGrossCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccountsNotesAndLoansReceivableLineItems_019e26c1-ff34-7cbe-ad54-60eee77b97d3" xlink:to="loc_us-gaap_AccountsReceivableGrossCurrent_019e26c1-ff34-707f-b5a0-a0aa5b1d6a68" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllowanceForDoubtfulAccountsReceivable_019e26c1-ff34-7f17-84b2-01b416db9527" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AllowanceForDoubtfulAccountsReceivable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccountsNotesAndLoansReceivableLineItems_019e26c1-ff34-7cbe-ad54-60eee77b97d3" xlink:to="loc_us-gaap_AllowanceForDoubtfulAccountsReceivable_019e26c1-ff34-7f17-84b2-01b416db9527" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsReceivableNetCurrent_019e26c1-ff34-759c-80cc-4b6698c7888a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccountsReceivableNetCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccountsNotesAndLoansReceivableLineItems_019e26c1-ff34-7cbe-ad54-60eee77b97d3" xlink:to="loc_us-gaap_AccountsReceivableNetCurrent_019e26c1-ff34-759c-80cc-4b6698c7888a" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.five9.com/role/FinancialStatementComponentsNarrativeDetails" xlink:type="simple" xlink:href="fivn-20260630.xsd#FinancialStatementComponentsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.five9.com/role/FinancialStatementComponentsNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskLineItems_019e26c1-ff34-7c74-9ff6-15a36c27a25c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTable_019e26c1-ff34-7443-b462-4b918fff74bb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_019e26c1-ff34-7c74-9ff6-15a36c27a25c" xlink:to="loc_us-gaap_ConcentrationRiskTable_019e26c1-ff34-7443-b462-4b918fff74bb" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MajorCustomersAxis_019e26c1-ff34-733d-8d44-6a1b9525873c" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MajorCustomersAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_019e26c1-ff34-7443-b462-4b918fff74bb" xlink:to="loc_srt_MajorCustomersAxis_019e26c1-ff34-733d-8d44-6a1b9525873c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_NameOfMajorCustomerDomain_019e26c1-ff34-733d-8d44-6a1b9525873c_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_NameOfMajorCustomerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_MajorCustomersAxis_019e26c1-ff34-733d-8d44-6a1b9525873c" xlink:to="loc_srt_NameOfMajorCustomerDomain_019e26c1-ff34-733d-8d44-6a1b9525873c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_NameOfMajorCustomerDomain_019e26c1-ff34-7352-b545-6c0674d35875" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_NameOfMajorCustomerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_MajorCustomersAxis_019e26c1-ff34-733d-8d44-6a1b9525873c" xlink:to="loc_srt_NameOfMajorCustomerDomain_019e26c1-ff34-7352-b545-6c0674d35875" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_OneClientMember_019e26c1-ff34-7c60-ae26-7e55ff17b6d4" xlink:href="fivn-20260630.xsd#fivn_OneClientMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_019e26c1-ff34-7352-b545-6c0674d35875" xlink:to="loc_fivn_OneClientMember_019e26c1-ff34-7c60-ae26-7e55ff17b6d4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019e26c1-ff34-7b58-8e79-0472590c306f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskByBenchmarkAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_019e26c1-ff34-7443-b462-4b918fff74bb" xlink:to="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019e26c1-ff34-7b58-8e79-0472590c306f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019e26c1-ff34-7b58-8e79-0472590c306f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019e26c1-ff34-7b58-8e79-0472590c306f" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019e26c1-ff34-7b58-8e79-0472590c306f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019e26c1-ff34-7451-9722-93b9368e1424" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019e26c1-ff34-7b58-8e79-0472590c306f" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019e26c1-ff34-7451-9722-93b9368e1424" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsReceivableMember_019e26c1-ff34-7bde-8880-99adbaf542f5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccountsReceivableMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019e26c1-ff34-7451-9722-93b9368e1424" xlink:to="loc_us-gaap_AccountsReceivableMember_019e26c1-ff34-7bde-8880-99adbaf542f5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByTypeAxis_019e26c1-ff34-7f53-aba2-4393bd6b02ec" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_019e26c1-ff34-7443-b462-4b918fff74bb" xlink:to="loc_us-gaap_ConcentrationRiskByTypeAxis_019e26c1-ff34-7f53-aba2-4393bd6b02ec" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_019e26c1-ff34-7f53-aba2-4393bd6b02ec_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_019e26c1-ff34-7f53-aba2-4393bd6b02ec" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_019e26c1-ff34-7f53-aba2-4393bd6b02ec_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_019e26c1-ff34-7954-a99a-d9fa32dc6d8e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_019e26c1-ff34-7f53-aba2-4393bd6b02ec" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_019e26c1-ff34-7954-a99a-d9fa32dc6d8e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerConcentrationRiskMember_019e26c1-ff34-7bcd-82bb-9c8d994f125b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CustomerConcentrationRiskMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskTypeDomain_019e26c1-ff34-7954-a99a-d9fa32dc6d8e" xlink:to="loc_us-gaap_CustomerConcentrationRiskMember_019e26c1-ff34-7bcd-82bb-9c8d994f125b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskPercentage1_019e26c1-ff34-791c-86d3-31224a577337" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConcentrationRiskPercentage1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_019e26c1-ff34-7c74-9ff6-15a36c27a25c" xlink:to="loc_us-gaap_ConcentrationRiskPercentage1_019e26c1-ff34-791c-86d3-31224a577337" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Depreciation_019e26c1-ff34-73d2-917d-47a6551315c0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Depreciation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_019e26c1-ff34-7c74-9ff6-15a36c27a25c" xlink:to="loc_us-gaap_Depreciation_019e26c1-ff34-73d2-917d-47a6551315c0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TangibleAssetImpairmentCharges_019e26c1-ff34-7e8c-b252-e1764a06a7f7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TangibleAssetImpairmentCharges"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_019e26c1-ff34-7c74-9ff6-15a36c27a25c" xlink:to="loc_us-gaap_TangibleAssetImpairmentCharges_019e26c1-ff34-7e8c-b252-e1764a06a7f7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_FinancingArrangementCommitmentAmount_019e26c1-ff34-756b-9d7f-e19fcc78fb81" xlink:href="fivn-20260630.xsd#fivn_FinancingArrangementCommitmentAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_019e26c1-ff34-7c74-9ff6-15a36c27a25c" xlink:to="loc_fivn_FinancingArrangementCommitmentAmount_019e26c1-ff34-756b-9d7f-e19fcc78fb81" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_FinancingArrangementTerm_019e26c1-ff34-7220-9e89-83226f7dae98" xlink:href="fivn-20260630.xsd#fivn_FinancingArrangementTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_019e26c1-ff34-7c74-9ff6-15a36c27a25c" xlink:to="loc_fivn_FinancingArrangementTerm_019e26c1-ff34-7220-9e89-83226f7dae98" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_FinancingArrangementNetPresentValue_019e26c1-ff34-7d08-9f17-f0a1c306f6bd" xlink:href="fivn-20260630.xsd#fivn_FinancingArrangementNetPresentValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_019e26c1-ff34-7c74-9ff6-15a36c27a25c" xlink:to="loc_fivn_FinancingArrangementNetPresentValue_019e26c1-ff34-7d08-9f17-f0a1c306f6bd" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_FinancingAssetsCurrent_019fb833-dafa-7ec8-9d76-2bd81f9670e9" xlink:href="fivn-20260630.xsd#fivn_FinancingAssetsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_019e26c1-ff34-7c74-9ff6-15a36c27a25c" xlink:to="loc_fivn_FinancingAssetsCurrent_019fb833-dafa-7ec8-9d76-2bd81f9670e9" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_FinancingAssetsNoncurrent_019fb834-4063-71e7-96b5-670bc9494f2f" xlink:href="fivn-20260630.xsd#fivn_FinancingAssetsNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_019e26c1-ff34-7c74-9ff6-15a36c27a25c" xlink:to="loc_fivn_FinancingAssetsNoncurrent_019fb834-4063-71e7-96b5-670bc9494f2f" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_FinancingLiabilityCurrent_019e26c1-ff34-74f7-ad9e-a62d3394359b" xlink:href="fivn-20260630.xsd#fivn_FinancingLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_019e26c1-ff34-7c74-9ff6-15a36c27a25c" xlink:to="loc_fivn_FinancingLiabilityCurrent_019e26c1-ff34-74f7-ad9e-a62d3394359b" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_FinancingLiabilityNoncurrent_019e26c1-ff34-7c5a-9530-3de9afa6535b" xlink:href="fivn-20260630.xsd#fivn_FinancingLiabilityNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_019e26c1-ff34-7c74-9ff6-15a36c27a25c" xlink:to="loc_fivn_FinancingLiabilityNoncurrent_019e26c1-ff34-7c5a-9530-3de9afa6535b" xlink:type="arc" order="9"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.five9.com/role/FinancialStatementComponentsScheduleofPropertyandEquipmentDetails" xlink:type="simple" xlink:href="fivn-20260630.xsd#FinancialStatementComponentsScheduleofPropertyandEquipmentDetails"/>
  <link:definitionLink xlink:role="http://www.five9.com/role/FinancialStatementComponentsScheduleofPropertyandEquipmentDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019e26c1-ff34-7f2f-9cc0-baa8712b8f3d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_019e26c1-ff34-7289-a830-d732c58c3c56" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019e26c1-ff34-7f2f-9cc0-baa8712b8f3d" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_019e26c1-ff34-7289-a830-d732c58c3c56" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019e26c1-ff34-759b-a8ac-d2ded3d1634a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_019e26c1-ff34-7289-a830-d732c58c3c56" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019e26c1-ff34-759b-a8ac-d2ded3d1634a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e26c1-ff34-759b-a8ac-d2ded3d1634a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019e26c1-ff34-759b-a8ac-d2ded3d1634a" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e26c1-ff34-759b-a8ac-d2ded3d1634a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e26c1-ff34-75a2-a8f1-fe7274d7ca33" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019e26c1-ff34-759b-a8ac-d2ded3d1634a" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e26c1-ff34-75a2-a8f1-fe7274d7ca33" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ComputerEquipmentMember_019e26c1-ff34-7205-9d70-95279e68ead7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ComputerEquipmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e26c1-ff34-75a2-a8f1-fe7274d7ca33" xlink:to="loc_us-gaap_ComputerEquipmentMember_019e26c1-ff34-7205-9d70-95279e68ead7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_ComputerSoftwareMember_019e26c1-ff34-707d-951d-f83e70e49823" xlink:href="fivn-20260630.xsd#fivn_ComputerSoftwareMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e26c1-ff34-75a2-a8f1-fe7274d7ca33" xlink:to="loc_fivn_ComputerSoftwareMember_019e26c1-ff34-707d-951d-f83e70e49823" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SoftwareDevelopmentMember_019e26c1-ff34-7cdf-b51b-f896da8693f0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SoftwareDevelopmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e26c1-ff34-75a2-a8f1-fe7274d7ca33" xlink:to="loc_us-gaap_SoftwareDevelopmentMember_019e26c1-ff34-7cdf-b51b-f896da8693f0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FurnitureAndFixturesMember_019e26c1-ff34-7122-b595-b604240b731a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FurnitureAndFixturesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e26c1-ff34-75a2-a8f1-fe7274d7ca33" xlink:to="loc_us-gaap_FurnitureAndFixturesMember_019e26c1-ff34-7122-b595-b604240b731a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseholdImprovementsMember_019e26c1-ff34-77cf-a285-2be1b15fccbf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LeaseholdImprovementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019e26c1-ff34-75a2-a8f1-fe7274d7ca33" xlink:to="loc_us-gaap_LeaseholdImprovementsMember_019e26c1-ff34-77cf-a285-2be1b15fccbf" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetBeforeAccumulatedDepreciationAndAmortization_019e26c1-ff34-7ba4-b2a0-362a146ea4a5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetBeforeAccumulatedDepreciationAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019e26c1-ff34-7f2f-9cc0-baa8712b8f3d" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetBeforeAccumulatedDepreciationAndAmortization_019e26c1-ff34-7ba4-b2a0-362a146ea4a5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAccumulatedDepreciationAndAmortization_019e26c1-ff34-7d54-949e-f4105c1005a5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAccumulatedDepreciationAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019e26c1-ff34-7f2f-9cc0-baa8712b8f3d" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAccumulatedDepreciationAndAmortization_019e26c1-ff34-7d54-949e-f4105c1005a5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization_019e26c1-ff34-71e7-b2e3-7948f7c4aebf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019e26c1-ff34-7f2f-9cc0-baa8712b8f3d" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization_019e26c1-ff34-71e7-b2e3-7948f7c4aebf" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.five9.com/role/GoodwillandIntangibleAssetsScheduleofIntangibleAssetsDetails" xlink:type="simple" xlink:href="fivn-20260630.xsd#GoodwillandIntangibleAssetsScheduleofIntangibleAssetsDetails"/>
  <link:definitionLink xlink:role="http://www.five9.com/role/GoodwillandIntangibleAssetsScheduleofIntangibleAssetsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems_019e26c1-ff34-788e-a514-0e23b5bbd412" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAcquiredFiniteLivedIntangibleAssetByMajorClassTable_019e26c1-ff34-76db-a38f-404aeb661638" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfAcquiredFiniteLivedIntangibleAssetByMajorClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems_019e26c1-ff34-788e-a514-0e23b5bbd412" xlink:to="loc_us-gaap_ScheduleOfAcquiredFiniteLivedIntangibleAssetByMajorClassTable_019e26c1-ff34-76db-a38f-404aeb661638" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019e26c1-ff34-710e-8a2b-7083b18b5cfe" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAcquiredFiniteLivedIntangibleAssetByMajorClassTable_019e26c1-ff34-76db-a38f-404aeb661638" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019e26c1-ff34-710e-8a2b-7083b18b5cfe" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019e26c1-ff34-710e-8a2b-7083b18b5cfe_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019e26c1-ff34-710e-8a2b-7083b18b5cfe" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019e26c1-ff34-710e-8a2b-7083b18b5cfe_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019e26c1-ff34-7ac8-bbeb-021ed8663a42" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019e26c1-ff34-710e-8a2b-7083b18b5cfe" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019e26c1-ff34-7ac8-bbeb-021ed8663a42" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DevelopedTechnologyRightsMember_019e26c1-ff34-7591-a1ad-37a3515cde8e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DevelopedTechnologyRightsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019e26c1-ff34-7ac8-bbeb-021ed8663a42" xlink:to="loc_us-gaap_DevelopedTechnologyRightsMember_019e26c1-ff34-7591-a1ad-37a3515cde8e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmploymentContractsMember_019e26c1-ff34-71ec-90bb-251ef5fb1152" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmploymentContractsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019e26c1-ff34-7ac8-bbeb-021ed8663a42" xlink:to="loc_us-gaap_EmploymentContractsMember_019e26c1-ff34-71ec-90bb-251ef5fb1152" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_019e26c1-ff34-76cf-9ee3-8b1dc68ade68" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CustomerRelationshipsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019e26c1-ff34-7ac8-bbeb-021ed8663a42" xlink:to="loc_us-gaap_CustomerRelationshipsMember_019e26c1-ff34-76cf-9ee3-8b1dc68ade68" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TrademarksMember_019e26c1-ff34-7234-912c-0ca3cafa38f5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TrademarksMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019e26c1-ff34-7ac8-bbeb-021ed8663a42" xlink:to="loc_us-gaap_TrademarksMember_019e26c1-ff34-7234-912c-0ca3cafa38f5" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsGross_019e26c1-ff34-74ee-aca8-3f81dbbf5c1e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems_019e26c1-ff34-788e-a514-0e23b5bbd412" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsGross_019e26c1-ff34-74ee-aca8-3f81dbbf5c1e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_019e26c1-ff34-75b3-bf6e-29b4c2f4b044" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems_019e26c1-ff34-788e-a514-0e23b5bbd412" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_019e26c1-ff34-75b3-bf6e-29b4c2f4b044" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsNet_019e26c1-ff34-7fe6-90d2-836d15d91a88" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems_019e26c1-ff34-788e-a514-0e23b5bbd412" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsNet_019e26c1-ff34-7fe6-90d2-836d15d91a88" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_019e26c1-ff34-76a3-a32e-ef880ba691f0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FiniteLivedIntangibleAssetUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems_019e26c1-ff34-788e-a514-0e23b5bbd412" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_019e26c1-ff34-76a3-a32e-ef880ba691f0" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.five9.com/role/DebtNarrativeDetails" xlink:type="simple" xlink:href="fivn-20260630.xsd#DebtNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.five9.com/role/DebtNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019e26c2-0007-7653-b75d-4e7ea11c03f1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019e26c2-0007-7918-8baa-2bf5177e3cb7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e26c2-0007-7653-b75d-4e7ea11c03f1" xlink:to="loc_us-gaap_DebtInstrumentTable_019e26c2-0007-7918-8baa-2bf5177e3cb7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019e26c2-0007-7e92-8631-87e8bb8f5d97" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e26c2-0007-7918-8baa-2bf5177e3cb7" xlink:to="loc_us-gaap_DebtInstrumentAxis_019e26c2-0007-7e92-8631-87e8bb8f5d97" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e26c2-0007-7e92-8631-87e8bb8f5d97_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e26c2-0007-7e92-8631-87e8bb8f5d97" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e26c2-0007-7e92-8631-87e8bb8f5d97_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e26c2-0007-7597-98d4-4520c91f3789" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e26c2-0007-7e92-8631-87e8bb8f5d97" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e26c2-0007-7597-98d4-4520c91f3789" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_ConvertibleSeniorNotesDue2029Member_019e26c2-0007-771f-a1ed-4fa5f0f91c7c" xlink:href="fivn-20260630.xsd#fivn_ConvertibleSeniorNotesDue2029Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e26c2-0007-7597-98d4-4520c91f3789" xlink:to="loc_fivn_ConvertibleSeniorNotesDue2029Member_019e26c2-0007-771f-a1ed-4fa5f0f91c7c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_ConvertibleSeniorNotesDue2025Member_019e26c2-0007-7fc5-aaa0-bf8164c0ec8e" xlink:href="fivn-20260630.xsd#fivn_ConvertibleSeniorNotesDue2025Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e26c2-0007-7597-98d4-4520c91f3789" xlink:to="loc_fivn_ConvertibleSeniorNotesDue2025Member_019e26c2-0007-7fc5-aaa0-bf8164c0ec8e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019e26c2-0007-7561-b7ff-8b4dbed37bac" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e26c2-0007-7918-8baa-2bf5177e3cb7" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019e26c2-0007-7561-b7ff-8b4dbed37bac" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e26c2-0007-7561-b7ff-8b4dbed37bac_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e26c2-0007-7561-b7ff-8b4dbed37bac" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e26c2-0007-7561-b7ff-8b4dbed37bac_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e26c2-0007-7bf2-88d2-a0ce187ece02" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e26c2-0007-7561-b7ff-8b4dbed37bac" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e26c2-0007-7bf2-88d2-a0ce187ece02" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtMember_019e26c2-0007-7a0e-bcc8-05b1387a1ee2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConvertibleDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019e26c2-0007-7bf2-88d2-a0ce187ece02" xlink:to="loc_us-gaap_ConvertibleDebtMember_019e26c2-0007-7a0e-bcc8-05b1387a1ee2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_019e26c2-0007-7e12-8666-f7fd717154dc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsidiarySaleOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e26c2-0007-7918-8baa-2bf5177e3cb7" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_019e26c2-0007-7e12-8666-f7fd717154dc" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019e26c2-0007-7e12-8666-f7fd717154dc_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019e26c2-0007-7e12-8666-f7fd717154dc" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019e26c2-0007-7e12-8666-f7fd717154dc_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019e26c2-0007-7151-975b-3626680970c8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019e26c2-0007-7e12-8666-f7fd717154dc" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019e26c2-0007-7151-975b-3626680970c8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrivatePlacementMember_019e26c2-0007-70d7-be93-76774d5856c9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PrivatePlacementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019e26c2-0007-7151-975b-3626680970c8" xlink:to="loc_us-gaap_PrivatePlacementMember_019e26c2-0007-70d7-be93-76774d5856c9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_DebtInstrumentConvertibleTermAxis_019e26c2-0007-7f73-96d5-3576f9aa87be" xlink:href="fivn-20260630.xsd#fivn_DebtInstrumentConvertibleTermAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e26c2-0007-7918-8baa-2bf5177e3cb7" xlink:to="loc_fivn_DebtInstrumentConvertibleTermAxis_019e26c2-0007-7f73-96d5-3576f9aa87be" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_DebtInstrumentConvertibleTermDomain_019e26c2-0007-7f73-96d5-3576f9aa87be_default" xlink:href="fivn-20260630.xsd#fivn_DebtInstrumentConvertibleTermDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_fivn_DebtInstrumentConvertibleTermAxis_019e26c2-0007-7f73-96d5-3576f9aa87be" xlink:to="loc_fivn_DebtInstrumentConvertibleTermDomain_019e26c2-0007-7f73-96d5-3576f9aa87be_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_DebtInstrumentConvertibleTermDomain_019e26c2-0007-7606-83ff-416f27fddb61" xlink:href="fivn-20260630.xsd#fivn_DebtInstrumentConvertibleTermDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_fivn_DebtInstrumentConvertibleTermAxis_019e26c2-0007-7f73-96d5-3576f9aa87be" xlink:to="loc_fivn_DebtInstrumentConvertibleTermDomain_019e26c2-0007-7606-83ff-416f27fddb61" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_DebtInstrumentConvertibleTermOneMember_019e26c2-0007-7a32-87be-564739a52766" xlink:href="fivn-20260630.xsd#fivn_DebtInstrumentConvertibleTermOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fivn_DebtInstrumentConvertibleTermDomain_019e26c2-0007-7606-83ff-416f27fddb61" xlink:to="loc_fivn_DebtInstrumentConvertibleTermOneMember_019e26c2-0007-7a32-87be-564739a52766" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_DebtInstrumentConvertibleTermTwoMember_019e26c2-0007-789d-bd44-bb31813006bd" xlink:href="fivn-20260630.xsd#fivn_DebtInstrumentConvertibleTermTwoMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fivn_DebtInstrumentConvertibleTermDomain_019e26c2-0007-7606-83ff-416f27fddb61" xlink:to="loc_fivn_DebtInstrumentConvertibleTermTwoMember_019e26c2-0007-789d-bd44-bb31813006bd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentRedemptionPeriodAxis_019e26c2-0007-793e-ba8e-93960906ad5d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentRedemptionPeriodAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e26c2-0007-7918-8baa-2bf5177e3cb7" xlink:to="loc_us-gaap_DebtInstrumentRedemptionPeriodAxis_019e26c2-0007-793e-ba8e-93960906ad5d" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentRedemptionPeriodDomain_019e26c2-0007-793e-ba8e-93960906ad5d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentRedemptionPeriodDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentRedemptionPeriodAxis_019e26c2-0007-793e-ba8e-93960906ad5d" xlink:to="loc_us-gaap_DebtInstrumentRedemptionPeriodDomain_019e26c2-0007-793e-ba8e-93960906ad5d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentRedemptionPeriodDomain_019e26c2-0007-7c19-b11e-4c120a2790da" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentRedemptionPeriodDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentRedemptionPeriodAxis_019e26c2-0007-793e-ba8e-93960906ad5d" xlink:to="loc_us-gaap_DebtInstrumentRedemptionPeriodDomain_019e26c2-0007-7c19-b11e-4c120a2790da" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentRedemptionPeriodOneMember_019e26c2-0007-70ce-938a-96f45faa85f9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentRedemptionPeriodOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentRedemptionPeriodDomain_019e26c2-0007-7c19-b11e-4c120a2790da" xlink:to="loc_us-gaap_DebtInstrumentRedemptionPeriodOneMember_019e26c2-0007-70ce-938a-96f45faa85f9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentRedemptionPeriodThreeMember_019e26c2-0007-7199-9c68-fec82c22a867" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentRedemptionPeriodThreeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentRedemptionPeriodDomain_019e26c2-0007-7c19-b11e-4c120a2790da" xlink:to="loc_us-gaap_DebtInstrumentRedemptionPeriodThreeMember_019e26c2-0007-7199-9c68-fec82c22a867" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_019e26c2-0007-7ff2-a884-19369ecc672f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e26c2-0007-7653-b75d-4e7ea11c03f1" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_019e26c2-0007-7ff2-a884-19369ecc672f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_DebtInstrumentOptionToPurchaseAdditionalDebtThroughPrivateOffering_019e26c2-0007-7142-9e07-664b816b0c93" xlink:href="fivn-20260630.xsd#fivn_DebtInstrumentOptionToPurchaseAdditionalDebtThroughPrivateOffering"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e26c2-0007-7653-b75d-4e7ea11c03f1" xlink:to="loc_fivn_DebtInstrumentOptionToPurchaseAdditionalDebtThroughPrivateOffering_019e26c2-0007-7142-9e07-664b816b0c93" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019e26c2-0007-7603-80b5-735878207551" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e26c2-0007-7653-b75d-4e7ea11c03f1" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019e26c2-0007-7603-80b5-735878207551" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfDebt_019e26c2-0007-705e-9038-6a27c91264d0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProceedsFromIssuanceOfDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e26c2-0007-7653-b75d-4e7ea11c03f1" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfDebt_019e26c2-0007-705e-9038-6a27c91264d0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentConvertibleConversionRatio1_019e26c2-0007-7d54-93f8-83a9c99caaab" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentConvertibleConversionRatio1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e26c2-0007-7653-b75d-4e7ea11c03f1" xlink:to="loc_us-gaap_DebtInstrumentConvertibleConversionRatio1_019e26c2-0007-7d54-93f8-83a9c99caaab" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentConvertibleConversionPrice1_019e26c2-0007-7eff-b71c-2057f262c52b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentConvertibleConversionPrice1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e26c2-0007-7653-b75d-4e7ea11c03f1" xlink:to="loc_us-gaap_DebtInstrumentConvertibleConversionPrice1_019e26c2-0007-7eff-b71c-2057f262c52b" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_DebtInstrumentPremiumOnConversionPriceInRelationToClosingPrice_019e26c2-0007-7300-8263-ee43af1da076" xlink:href="fivn-20260630.xsd#fivn_DebtInstrumentPremiumOnConversionPriceInRelationToClosingPrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e26c2-0007-7653-b75d-4e7ea11c03f1" xlink:to="loc_fivn_DebtInstrumentPremiumOnConversionPriceInRelationToClosingPrice_019e26c2-0007-7300-8263-ee43af1da076" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_ClosingPriceOfCommonStock_019e26c2-0007-773f-90dd-b45453b75472" xlink:href="fivn-20260630.xsd#fivn_ClosingPriceOfCommonStock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e26c2-0007-7653-b75d-4e7ea11c03f1" xlink:to="loc_fivn_ClosingPriceOfCommonStock_019e26c2-0007-773f-90dd-b45453b75472" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentConvertibleThresholdTradingDays_019e26c2-0007-730a-afc6-247e16d2a640" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentConvertibleThresholdTradingDays"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e26c2-0007-7653-b75d-4e7ea11c03f1" xlink:to="loc_us-gaap_DebtInstrumentConvertibleThresholdTradingDays_019e26c2-0007-730a-afc6-247e16d2a640" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentConvertibleThresholdConsecutiveTradingDays1_019e26c2-0007-7b35-83f8-3afbca4c6c11" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentConvertibleThresholdConsecutiveTradingDays1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e26c2-0007-7653-b75d-4e7ea11c03f1" xlink:to="loc_us-gaap_DebtInstrumentConvertibleThresholdConsecutiveTradingDays1_019e26c2-0007-7b35-83f8-3afbca4c6c11" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentConvertibleThresholdPercentageOfStockPriceTrigger_019e26c2-0007-79e0-8c60-d2dd373f4305" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentConvertibleThresholdPercentageOfStockPriceTrigger"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e26c2-0007-7653-b75d-4e7ea11c03f1" xlink:to="loc_us-gaap_DebtInstrumentConvertibleThresholdPercentageOfStockPriceTrigger_019e26c2-0007-79e0-8c60-d2dd373f4305" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentRedemptionPricePercentage_019e26c2-0007-75c4-bae3-9ed44e40506f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentRedemptionPricePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e26c2-0007-7653-b75d-4e7ea11c03f1" xlink:to="loc_us-gaap_DebtInstrumentRedemptionPricePercentage_019e26c2-0007-75c4-bae3-9ed44e40506f" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_SharePriceOfCommonStockWithConversionPremium_019e26c2-0007-7cca-b0ac-f18d4e9948a5" xlink:href="fivn-20260630.xsd#fivn_SharePriceOfCommonStockWithConversionPremium"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e26c2-0007-7653-b75d-4e7ea11c03f1" xlink:to="loc_fivn_SharePriceOfCommonStockWithConversionPremium_019e26c2-0007-7cca-b0ac-f18d4e9948a5" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_DebtInstrumentRedemptionMinimumAggregatePrincipalAmountOutstandingRequiredForRedemption_019e26c2-0007-7719-a3ea-5eaed856c979" xlink:href="fivn-20260630.xsd#fivn_DebtInstrumentRedemptionMinimumAggregatePrincipalAmountOutstandingRequiredForRedemption"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e26c2-0007-7653-b75d-4e7ea11c03f1" xlink:to="loc_fivn_DebtInstrumentRedemptionMinimumAggregatePrincipalAmountOutstandingRequiredForRedemption_019e26c2-0007-7719-a3ea-5eaed856c979" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentSinkingFundPayment_019e26c2-0007-7560-84fe-47508e1ae56a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentSinkingFundPayment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e26c2-0007-7653-b75d-4e7ea11c03f1" xlink:to="loc_us-gaap_DebtInstrumentSinkingFundPayment_019e26c2-0007-7560-84fe-47508e1ae56a" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_019e26c2-0007-75d8-8801-5817f86e094c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentInterestRateEffectivePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e26c2-0007-7653-b75d-4e7ea11c03f1" xlink:to="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_019e26c2-0007-75d8-8801-5817f86e094c" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentConvertibleStockPriceTrigger_019e26c2-0007-7523-b135-dcb79d7cea30" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentConvertibleStockPriceTrigger"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e26c2-0007-7653-b75d-4e7ea11c03f1" xlink:to="loc_us-gaap_DebtInstrumentConvertibleStockPriceTrigger_019e26c2-0007-7523-b135-dcb79d7cea30" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_DebtInstrumentConvertibleCappedCallTransactionsCapPrice_019e26c2-0007-7741-b61f-1e72ddff80ec" xlink:href="fivn-20260630.xsd#fivn_DebtInstrumentConvertibleCappedCallTransactionsCapPrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e26c2-0007-7653-b75d-4e7ea11c03f1" xlink:to="loc_fivn_DebtInstrumentConvertibleCappedCallTransactionsCapPrice_019e26c2-0007-7741-b61f-1e72ddff80ec" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_DebtInstrumentCappedCallTransactionsSharesCoverage_019e26c2-0007-7184-a329-d93fa737eb73" xlink:href="fivn-20260630.xsd#fivn_DebtInstrumentCappedCallTransactionsSharesCoverage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e26c2-0007-7653-b75d-4e7ea11c03f1" xlink:to="loc_fivn_DebtInstrumentCappedCallTransactionsSharesCoverage_019e26c2-0007-7184-a329-d93fa737eb73" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ExtinguishmentOfDebtAmount_019e26c2-0007-7c55-9f21-28e00b4943f4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ExtinguishmentOfDebtAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e26c2-0007-7653-b75d-4e7ea11c03f1" xlink:to="loc_us-gaap_ExtinguishmentOfDebtAmount_019e26c2-0007-7c55-9f21-28e00b4943f4" xlink:type="arc" order="19"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.five9.com/role/DebtScheduleofNetCarryingAmountoftheLiabilityComponentoftheNotesDetails" xlink:type="simple" xlink:href="fivn-20260630.xsd#DebtScheduleofNetCarryingAmountoftheLiabilityComponentoftheNotesDetails"/>
  <link:definitionLink xlink:role="http://www.five9.com/role/DebtScheduleofNetCarryingAmountoftheLiabilityComponentoftheNotesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019e26c2-0007-748c-82c9-232d14543c02" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019e26c2-0007-70c6-b68f-4007e66fa772" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e26c2-0007-748c-82c9-232d14543c02" xlink:to="loc_us-gaap_DebtInstrumentTable_019e26c2-0007-70c6-b68f-4007e66fa772" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019e26c2-0007-7520-8bb2-e7c4668371e3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e26c2-0007-70c6-b68f-4007e66fa772" xlink:to="loc_us-gaap_DebtInstrumentAxis_019e26c2-0007-7520-8bb2-e7c4668371e3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e26c2-0007-7520-8bb2-e7c4668371e3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e26c2-0007-7520-8bb2-e7c4668371e3" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e26c2-0007-7520-8bb2-e7c4668371e3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e26c2-0007-7e85-b11c-d16aeec2fff1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e26c2-0007-7520-8bb2-e7c4668371e3" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e26c2-0007-7e85-b11c-d16aeec2fff1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_ConvertibleSeniorNotesDue2029Member_019e26c2-0007-76fe-91e1-94f153a1cb6b" xlink:href="fivn-20260630.xsd#fivn_ConvertibleSeniorNotesDue2029Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e26c2-0007-7e85-b11c-d16aeec2fff1" xlink:to="loc_fivn_ConvertibleSeniorNotesDue2029Member_019e26c2-0007-76fe-91e1-94f153a1cb6b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019e26c2-0007-74f0-9048-9140fc737e3d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e26c2-0007-70c6-b68f-4007e66fa772" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019e26c2-0007-74f0-9048-9140fc737e3d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e26c2-0007-74f0-9048-9140fc737e3d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e26c2-0007-74f0-9048-9140fc737e3d" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e26c2-0007-74f0-9048-9140fc737e3d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e26c2-0007-78bf-8d44-a332d928c8ec" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e26c2-0007-74f0-9048-9140fc737e3d" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e26c2-0007-78bf-8d44-a332d928c8ec" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtMember_019e26c2-0007-7939-a32f-c12dae113743" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConvertibleDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019e26c2-0007-78bf-8d44-a332d928c8ec" xlink:to="loc_us-gaap_ConvertibleDebtMember_019e26c2-0007-7939-a32f-c12dae113743" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentCarryingAmount_019e26c2-0007-711c-8dff-25a12dcf2f5e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentCarryingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e26c2-0007-748c-82c9-232d14543c02" xlink:to="loc_us-gaap_DebtInstrumentCarryingAmount_019e26c2-0007-711c-8dff-25a12dcf2f5e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredFinanceCostsGross_019e26c2-0007-709b-8617-33714720c42e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DeferredFinanceCostsGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e26c2-0007-748c-82c9-232d14543c02" xlink:to="loc_us-gaap_DeferredFinanceCostsGross_019e26c2-0007-709b-8617-33714720c42e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebt_019e26c2-0007-74d0-add7-13f1511b0a11" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e26c2-0007-748c-82c9-232d14543c02" xlink:to="loc_us-gaap_LongTermDebt_019e26c2-0007-74d0-add7-13f1511b0a11" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.five9.com/role/DebtScheduleofInterestExpenseRelatedtotheNotesDetails" xlink:type="simple" xlink:href="fivn-20260630.xsd#DebtScheduleofInterestExpenseRelatedtotheNotesDetails"/>
  <link:definitionLink xlink:role="http://www.five9.com/role/DebtScheduleofInterestExpenseRelatedtotheNotesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019e26c2-0007-7e61-b8af-fb5a5b6c62ce" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019e26c2-0007-78e6-ae53-269eb095aa3d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e26c2-0007-7e61-b8af-fb5a5b6c62ce" xlink:to="loc_us-gaap_DebtInstrumentTable_019e26c2-0007-78e6-ae53-269eb095aa3d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019e26c2-0007-7ea1-934f-2cced5c218ea" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e26c2-0007-78e6-ae53-269eb095aa3d" xlink:to="loc_us-gaap_DebtInstrumentAxis_019e26c2-0007-7ea1-934f-2cced5c218ea" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e26c2-0007-7ea1-934f-2cced5c218ea_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e26c2-0007-7ea1-934f-2cced5c218ea" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e26c2-0007-7ea1-934f-2cced5c218ea_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e26c2-0007-713c-b49b-fad9dcdb46f6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e26c2-0007-7ea1-934f-2cced5c218ea" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e26c2-0007-713c-b49b-fad9dcdb46f6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_ConvertibleSeniorNotesDue2029Member_019e26c2-0007-7e29-8ecd-84a175cf4ed3" xlink:href="fivn-20260630.xsd#fivn_ConvertibleSeniorNotesDue2029Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e26c2-0007-713c-b49b-fad9dcdb46f6" xlink:to="loc_fivn_ConvertibleSeniorNotesDue2029Member_019e26c2-0007-7e29-8ecd-84a175cf4ed3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019e26c2-0007-7b01-b402-3f42d812bfca" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019e26c2-0007-78e6-ae53-269eb095aa3d" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019e26c2-0007-7b01-b402-3f42d812bfca" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e26c2-0007-7b01-b402-3f42d812bfca_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e26c2-0007-7b01-b402-3f42d812bfca" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e26c2-0007-7b01-b402-3f42d812bfca_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e26c2-0007-7028-91e8-e83f323eac8d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e26c2-0007-7b01-b402-3f42d812bfca" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e26c2-0007-7028-91e8-e83f323eac8d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtMember_019e26c2-0007-715d-8347-29741f04c64a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConvertibleDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019e26c2-0007-7028-91e8-e83f323eac8d" xlink:to="loc_us-gaap_ConvertibleDebtMember_019e26c2-0007-715d-8347-29741f04c64a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpenseDebt_019e26c2-0007-7034-8fb2-243d9acaf6a3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestExpenseDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e26c2-0007-7e61-b8af-fb5a5b6c62ce" xlink:to="loc_us-gaap_InterestExpenseDebt_019e26c2-0007-7034-8fb2-243d9acaf6a3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AmortizationOfFinancingCosts_019e26c2-0007-740b-aab3-6391f8894ca6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AmortizationOfFinancingCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e26c2-0007-7e61-b8af-fb5a5b6c62ce" xlink:to="loc_us-gaap_AmortizationOfFinancingCosts_019e26c2-0007-740b-aab3-6391f8894ca6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpense_019e26c2-0007-7303-a0b5-b22790974144" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019e26c2-0007-7e61-b8af-fb5a5b6c62ce" xlink:to="loc_us-gaap_InterestExpense_019e26c2-0007-7303-a0b5-b22790974144" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.five9.com/role/StockholdersEquityShareRepurchaseProgramDetails" xlink:type="simple" xlink:href="fivn-20260630.xsd#StockholdersEquityShareRepurchaseProgramDetails"/>
  <link:definitionLink xlink:role="http://www.five9.com/role/StockholdersEquityShareRepurchaseProgramDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_srt_ShareRepurchaseProgramLineItems_019f0283-a672-7416-8c70-054bfa623ac4" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ShareRepurchaseProgramLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ShareRepurchaseProgramTable_019f0283-a672-7c17-a8da-96d08fd6313d" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ShareRepurchaseProgramTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_srt_ShareRepurchaseProgramLineItems_019f0283-a672-7416-8c70-054bfa623ac4" xlink:to="loc_srt_ShareRepurchaseProgramTable_019f0283-a672-7c17-a8da-96d08fd6313d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ShareRepurchaseProgramAxis_019f0283-a672-77a1-8ec5-6381f98b26dc" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ShareRepurchaseProgramAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_srt_ShareRepurchaseProgramTable_019f0283-a672-7c17-a8da-96d08fd6313d" xlink:to="loc_srt_ShareRepurchaseProgramAxis_019f0283-a672-77a1-8ec5-6381f98b26dc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ShareRepurchaseProgramDomain_019f0283-a672-77a1-8ec5-6381f98b26dc_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ShareRepurchaseProgramDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ShareRepurchaseProgramAxis_019f0283-a672-77a1-8ec5-6381f98b26dc" xlink:to="loc_srt_ShareRepurchaseProgramDomain_019f0283-a672-77a1-8ec5-6381f98b26dc_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ShareRepurchaseProgramDomain_019f0283-a672-72b1-9811-a7f5554879f5" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ShareRepurchaseProgramDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ShareRepurchaseProgramAxis_019f0283-a672-77a1-8ec5-6381f98b26dc" xlink:to="loc_srt_ShareRepurchaseProgramDomain_019f0283-a672-72b1-9811-a7f5554879f5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_A2025RepurchaseProgramMember_019f35f2-4cf8-7fec-95ef-5762ad2e3f3a" xlink:href="fivn-20260630.xsd#fivn_A2025RepurchaseProgramMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ShareRepurchaseProgramDomain_019f0283-a672-72b1-9811-a7f5554879f5" xlink:to="loc_fivn_A2025RepurchaseProgramMember_019f35f2-4cf8-7fec-95ef-5762ad2e3f3a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_A2025ASRProgramMember_019f6646-4a43-7496-a8fc-ad70a8c25c44" xlink:href="fivn-20260630.xsd#fivn_A2025ASRProgramMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ShareRepurchaseProgramDomain_019f0283-a672-72b1-9811-a7f5554879f5" xlink:to="loc_fivn_A2025ASRProgramMember_019f6646-4a43-7496-a8fc-ad70a8c25c44" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_A2026ASRProgramMember_019f6647-0837-728e-a7dd-95aa41de5184" xlink:href="fivn-20260630.xsd#fivn_A2026ASRProgramMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ShareRepurchaseProgramDomain_019f0283-a672-72b1-9811-a7f5554879f5" xlink:to="loc_fivn_A2026ASRProgramMember_019f6647-0837-728e-a7dd-95aa41de5184" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_A2026RepurchaseProgramMember_019f0283-a672-7dbe-b691-d075e9c78ea3" xlink:href="fivn-20260630.xsd#fivn_A2026RepurchaseProgramMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ShareRepurchaseProgramDomain_019f0283-a672-72b1-9811-a7f5554879f5" xlink:to="loc_fivn_A2026RepurchaseProgramMember_019f0283-a672-7dbe-b691-d075e9c78ea3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StockRepurchaseProgramAuthorizedAmount1_019e26c2-0007-7d60-a03d-6db3e576425a" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_StockRepurchaseProgramAuthorizedAmount1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ShareRepurchaseProgramLineItems_019f0283-a672-7416-8c70-054bfa623ac4" xlink:to="loc_srt_StockRepurchaseProgramAuthorizedAmount1_019e26c2-0007-7d60-a03d-6db3e576425a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedDuringPeriodValue_019e26c2-0007-75a9-b3e8-fb2fc518b3b7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockRepurchasedDuringPeriodValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ShareRepurchaseProgramLineItems_019f0283-a672-7416-8c70-054bfa623ac4" xlink:to="loc_us-gaap_StockRepurchasedDuringPeriodValue_019e26c2-0007-75a9-b3e8-fb2fc518b3b7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedDuringPeriodShares_019e26c2-0007-7f3a-aa37-60b807e9bc82" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockRepurchasedDuringPeriodShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ShareRepurchaseProgramLineItems_019f0283-a672-7416-8c70-054bfa623ac4" xlink:to="loc_us-gaap_StockRepurchasedDuringPeriodShares_019e26c2-0007-7f3a-aa37-60b807e9bc82" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockAcquiredAverageCostPerShare_019e26c2-0007-7fcc-a1ad-84fc907e0ee2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TreasuryStockAcquiredAverageCostPerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ShareRepurchaseProgramLineItems_019f0283-a672-7416-8c70-054bfa623ac4" xlink:to="loc_us-gaap_TreasuryStockAcquiredAverageCostPerShare_019e26c2-0007-7fcc-a1ad-84fc907e0ee2" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_StockRepurchaseProgramPercentageOfSharesToBePurchased_019e26c2-0007-7176-acf7-a6aa33b93727" xlink:href="fivn-20260630.xsd#fivn_StockRepurchaseProgramPercentageOfSharesToBePurchased"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ShareRepurchaseProgramLineItems_019f0283-a672-7416-8c70-054bfa623ac4" xlink:to="loc_fivn_StockRepurchaseProgramPercentageOfSharesToBePurchased_019e26c2-0007-7176-acf7-a6aa33b93727" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1_019f0283-a672-7fa8-827a-df5c8571a056" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ShareRepurchaseProgramLineItems_019f0283-a672-7416-8c70-054bfa623ac4" xlink:to="loc_us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1_019f0283-a672-7fa8-827a-df5c8571a056" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.five9.com/role/StockholdersEquityScheduleofCommonStockReservedforFutureIssuanceDetails" xlink:type="simple" xlink:href="fivn-20260630.xsd#StockholdersEquityScheduleofCommonStockReservedforFutureIssuanceDetails"/>
  <link:definitionLink xlink:role="http://www.five9.com/role/StockholdersEquityScheduleofCommonStockReservedforFutureIssuanceDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockLineItems_019e26c2-0007-7fbb-b19c-4a166e630922" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_019e26c2-0007-768f-8442-c24c5a532111" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfStockByClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_019e26c2-0007-7fbb-b19c-4a166e630922" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_019e26c2-0007-768f-8442-c24c5a532111" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019e26c2-0007-797d-b010-2c1ca1b0f39b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_019e26c2-0007-768f-8442-c24c5a532111" xlink:to="loc_us-gaap_AwardTypeAxis_019e26c2-0007-797d-b010-2c1ca1b0f39b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e26c2-0007-797d-b010-2c1ca1b0f39b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019e26c2-0007-797d-b010-2c1ca1b0f39b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e26c2-0007-797d-b010-2c1ca1b0f39b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e26c2-0007-7a9e-96b9-2f486a2bb73b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019e26c2-0007-797d-b010-2c1ca1b0f39b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e26c2-0007-7a9e-96b9-2f486a2bb73b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_019e26c2-0007-709d-8a77-cdce15a23730" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e26c2-0007-7a9e-96b9-2f486a2bb73b" xlink:to="loc_us-gaap_EmployeeStockOptionMember_019e26c2-0007-709d-8a77-cdce15a23730" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_019e26c2-0007-7435-a035-20d0517ffd49" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e26c2-0007-7a9e-96b9-2f486a2bb73b" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_019e26c2-0007-7435-a035-20d0517ffd49" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember_019e26c2-0007-7895-98ed-26755599823e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e26c2-0007-7a9e-96b9-2f486a2bb73b" xlink:to="loc_us-gaap_EmployeeStockMember_019e26c2-0007-7895-98ed-26755599823e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_019e26c2-0007-75c9-8b66-40558f3aa24b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_019e26c2-0007-768f-8442-c24c5a532111" xlink:to="loc_us-gaap_PlanNameAxis_019e26c2-0007-75c9-8b66-40558f3aa24b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_019e26c2-0007-75c9-8b66-40558f3aa24b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_019e26c2-0007-75c9-8b66-40558f3aa24b" xlink:to="loc_us-gaap_PlanNameDomain_019e26c2-0007-75c9-8b66-40558f3aa24b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_019e26c2-0007-737a-b1ae-bbd3733bd173" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_019e26c2-0007-75c9-8b66-40558f3aa24b" xlink:to="loc_us-gaap_PlanNameDomain_019e26c2-0007-737a-b1ae-bbd3733bd173" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_EquityIncentivePlan2014Member_019e26c2-0007-7212-9a8c-ca5967a214f4" xlink:href="fivn-20260630.xsd#fivn_EquityIncentivePlan2014Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_019e26c2-0007-737a-b1ae-bbd3733bd173" xlink:to="loc_fivn_EquityIncentivePlan2014Member_019e26c2-0007-7212-9a8c-ca5967a214f4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_A2026InducementPlanMember_019f358a-1885-7879-a5d1-3d9059707c7d" xlink:href="fivn-20260630.xsd#fivn_A2026InducementPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_019e26c2-0007-737a-b1ae-bbd3733bd173" xlink:to="loc_fivn_A2026InducementPlanMember_019f358a-1885-7879-a5d1-3d9059707c7d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_019e26c2-0007-74b4-9e54-c9450903869c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockCapitalSharesReservedForFutureIssuance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019e26c2-0007-7fbb-b19c-4a166e630922" xlink:to="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_019e26c2-0007-74b4-9e54-c9450903869c" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.five9.com/role/StockholdersEquityEquityIncentivePlansNarrativeDetails" xlink:type="simple" xlink:href="fivn-20260630.xsd#StockholdersEquityEquityIncentivePlansNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.five9.com/role/StockholdersEquityEquityIncentivePlansNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_srt_ShareRepurchaseProgramLineItems_019f35bf-ffec-776e-9ca6-b3909d06750f" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ShareRepurchaseProgramLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ShareRepurchaseProgramTable_019f35bf-ffec-7045-9d1f-0191bc826a69" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ShareRepurchaseProgramTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_srt_ShareRepurchaseProgramLineItems_019f35bf-ffec-776e-9ca6-b3909d06750f" xlink:to="loc_srt_ShareRepurchaseProgramTable_019f35bf-ffec-7045-9d1f-0191bc826a69" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_019f35bf-ffec-7b30-b8c4-25a80ea2e863" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_srt_ShareRepurchaseProgramTable_019f35bf-ffec-7045-9d1f-0191bc826a69" xlink:to="loc_us-gaap_PlanNameAxis_019f35bf-ffec-7b30-b8c4-25a80ea2e863" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_019f35bf-ffec-7b30-b8c4-25a80ea2e863_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_019f35bf-ffec-7b30-b8c4-25a80ea2e863" xlink:to="loc_us-gaap_PlanNameDomain_019f35bf-ffec-7b30-b8c4-25a80ea2e863_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_019f35bf-ffec-791d-97c3-1e12f19705ac" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_019f35bf-ffec-7b30-b8c4-25a80ea2e863" xlink:to="loc_us-gaap_PlanNameDomain_019f35bf-ffec-791d-97c3-1e12f19705ac" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_A2026InducementPlanMember_019f35bf-ffec-7b11-aa43-4191b3f6c106" xlink:href="fivn-20260630.xsd#fivn_A2026InducementPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_019f35bf-ffec-791d-97c3-1e12f19705ac" xlink:to="loc_fivn_A2026InducementPlanMember_019f35bf-ffec-7b11-aa43-4191b3f6c106" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019f35d7-7652-7d79-afc7-c502614ec9a4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_srt_ShareRepurchaseProgramTable_019f35bf-ffec-7045-9d1f-0191bc826a69" xlink:to="loc_us-gaap_AwardTypeAxis_019f35d7-7652-7d79-afc7-c502614ec9a4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019f35d7-7652-7d79-afc7-c502614ec9a4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019f35d7-7652-7d79-afc7-c502614ec9a4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019f35d7-7652-7d79-afc7-c502614ec9a4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019f35d7-7652-7b1a-a753-7caaf5733a85" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019f35d7-7652-7d79-afc7-c502614ec9a4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019f35d7-7652-7b1a-a753-7caaf5733a85" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_019f35d8-162d-7911-8d8b-039e6b5c723d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019f35d7-7652-7b1a-a753-7caaf5733a85" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_019f35d8-162d-7911-8d8b-039e6b5c723d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019f35db-9a41-7a8e-9ebb-16a4c2832e1e" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_srt_ShareRepurchaseProgramTable_019f35bf-ffec-7045-9d1f-0191bc826a69" xlink:to="loc_srt_RangeAxis_019f35db-9a41-7a8e-9ebb-16a4c2832e1e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019f35db-9a41-7a8e-9ebb-16a4c2832e1e_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019f35db-9a41-7a8e-9ebb-16a4c2832e1e" xlink:to="loc_srt_RangeMember_019f35db-9a41-7a8e-9ebb-16a4c2832e1e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019f35db-9a41-79cb-98da-a8998b66be81" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019f35db-9a41-7a8e-9ebb-16a4c2832e1e" xlink:to="loc_srt_RangeMember_019f35db-9a41-79cb-98da-a8998b66be81" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019f35db-c59d-7c04-b4c0-ff2121a2fcc6" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019f35db-9a41-79cb-98da-a8998b66be81" xlink:to="loc_srt_MinimumMember_019f35db-c59d-7c04-b4c0-ff2121a2fcc6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019f35db-ecb8-74d1-9ffb-b3b74785ac01" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019f35db-9a41-79cb-98da-a8998b66be81" xlink:to="loc_srt_MaximumMember_019f35db-ecb8-74d1-9ffb-b3b74785ac01" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_019f35bf-ffec-78ff-b86b-8f9de2b368f1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ShareRepurchaseProgramLineItems_019f35bf-ffec-776e-9ca6-b3909d06750f" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_019f35bf-ffec-78ff-b86b-8f9de2b368f1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_ShareBasedCompensationSharesPerUnitsGranted_019f35d6-3781-7fdc-b08b-a81ed77458a0" xlink:href="fivn-20260630.xsd#fivn_ShareBasedCompensationSharesPerUnitsGranted"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ShareRepurchaseProgramLineItems_019f35bf-ffec-776e-9ca6-b3909d06750f" xlink:to="loc_fivn_ShareBasedCompensationSharesPerUnitsGranted_019f35d6-3781-7fdc-b08b-a81ed77458a0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_019f35d9-f098-78f9-a219-dd820836dc59" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ShareRepurchaseProgramLineItems_019f35bf-ffec-776e-9ca6-b3909d06750f" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_019f35d9-f098-78f9-a219-dd820836dc59" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.five9.com/role/StockholdersEquityScheduleofStockOptionActivitiesDetails" xlink:type="simple" xlink:href="fivn-20260630.xsd#StockholdersEquityScheduleofStockOptionActivitiesDetails"/>
  <link:definitionLink xlink:role="http://www.five9.com/role/StockholdersEquityScheduleofStockOptionActivitiesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockLineItems_019e26c2-0007-75e6-9f1d-b7713e0ca065" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_019e26c2-0007-7d6d-b8b5-4723668fc60d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfStockByClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_019e26c2-0007-75e6-9f1d-b7713e0ca065" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_019e26c2-0007-7d6d-b8b5-4723668fc60d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019e26c2-0007-7e2d-9817-382d7642fa5a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_019e26c2-0007-7d6d-b8b5-4723668fc60d" xlink:to="loc_us-gaap_AwardTypeAxis_019e26c2-0007-7e2d-9817-382d7642fa5a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e26c2-0007-7e2d-9817-382d7642fa5a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019e26c2-0007-7e2d-9817-382d7642fa5a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e26c2-0007-7e2d-9817-382d7642fa5a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e26c2-0007-7101-9955-49b75b642565" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019e26c2-0007-7e2d-9817-382d7642fa5a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e26c2-0007-7101-9955-49b75b642565" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_019e26c2-0007-718b-8516-6ae14d65c467" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e26c2-0007-7101-9955-49b75b642565" xlink:to="loc_us-gaap_EmployeeStockOptionMember_019e26c2-0007-718b-8516-6ae14d65c467" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_019e26c2-0007-789f-b037-b097947a9993" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019e26c2-0007-75e6-9f1d-b7713e0ca065" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_019e26c2-0007-789f-b037-b097947a9993" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_019e26c2-0007-7f92-9879-5a8035626202" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_019e26c2-0007-789f-b037-b097947a9993" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_019e26c2-0007-7f92-9879-5a8035626202" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_019e26c2-0007-73a2-a567-2547ce67ee09" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_019e26c2-0007-789f-b037-b097947a9993" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_019e26c2-0007-73a2-a567-2547ce67ee09" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_019e26c2-0007-7970-ac47-ead233e701d7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_019e26c2-0007-789f-b037-b097947a9993" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_019e26c2-0007-7970-ac47-ead233e701d7" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriod_019e26c2-0007-7082-9356-278743eee99b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_019e26c2-0007-789f-b037-b097947a9993" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriod_019e26c2-0007-7082-9356-278743eee99b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_019e26c2-0007-7dce-97f2-9d34991bd9b9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber_019e26c2-0007-7632-867f-296c7a20fc12" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019e26c2-0007-75e6-9f1d-b7713e0ca065" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber_019e26c2-0007-7632-867f-296c7a20fc12" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber_019e26c2-0007-7b23-9139-cf4ba5789417" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019e26c2-0007-75e6-9f1d-b7713e0ca065" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber_019e26c2-0007-7b23-9139-cf4ba5789417" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_019e26c2-0007-7cbb-bd4d-824c7668416e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019e26c2-0007-75e6-9f1d-b7713e0ca065" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_019e26c2-0007-7cbb-bd4d-824c7668416e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_019e26c2-0007-7994-9f70-ccf622b7cc8b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_019e26c2-0007-7cbb-bd4d-824c7668416e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_019e26c2-0007-7994-9f70-ccf622b7cc8b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_019e26c2-0007-754e-b3d6-67129c852bc1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_019e26c2-0007-7cbb-bd4d-824c7668416e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_019e26c2-0007-754e-b3d6-67129c852bc1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice_019e26c2-0007-71ec-8572-d9c3ebbb9607" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_019e26c2-0007-7cbb-bd4d-824c7668416e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice_019e26c2-0007-71ec-8572-d9c3ebbb9607" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriodWeightedAverageExercisePrice_019e26c2-0007-7b3f-9c73-f944f85d9b1a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriodWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_019e26c2-0007-7cbb-bd4d-824c7668416e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriodWeightedAverageExercisePrice_019e26c2-0007-7b3f-9c73-f944f85d9b1a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_019e26c2-0007-7fd7-9389-e0c58a2c63ad" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice_019e26c2-0007-7cb4-9a58-ab7a2e6b6288" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019e26c2-0007-75e6-9f1d-b7713e0ca065" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice_019e26c2-0007-7cb4-9a58-ab7a2e6b6288" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice_019e26c2-0007-7a58-8e66-7b605bc055e5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019e26c2-0007-75e6-9f1d-b7713e0ca065" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice_019e26c2-0007-7a58-8e66-7b605bc055e5" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2_019e26c2-0007-7481-913e-24e0eae7cf07" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019e26c2-0007-75e6-9f1d-b7713e0ca065" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2_019e26c2-0007-7481-913e-24e0eae7cf07" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue_019e26c2-0007-7289-9f56-80e74b9f75d9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019e26c2-0007-75e6-9f1d-b7713e0ca065" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue_019e26c2-0007-7289-9f56-80e74b9f75d9" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestAbstract_019e26c2-0007-71d6-9ec2-f3c681eff789" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019e26c2-0007-75e6-9f1d-b7713e0ca065" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestAbstract_019e26c2-0007-71d6-9ec2-f3c681eff789" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageRemainingContractualTerm1_019e26c2-0007-7767-8fa6-47b3a6607a17" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageRemainingContractualTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestAbstract_019e26c2-0007-71d6-9ec2-f3c681eff789" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageRemainingContractualTerm1_019e26c2-0007-7767-8fa6-47b3a6607a17" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1_019e26c2-0007-7213-bdb2-61ca7729296c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestAbstract_019e26c2-0007-71d6-9ec2-f3c681eff789" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1_019e26c2-0007-7213-bdb2-61ca7729296c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue_019e26c2-0007-7ebd-a844-496756c6efba" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestAbstract_019e26c2-0007-71d6-9ec2-f3c681eff789" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue_019e26c2-0007-7ebd-a844-496756c6efba" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1_019e26c2-0007-7959-948c-3e842b9a0533" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestAbstract_019e26c2-0007-71d6-9ec2-f3c681eff789" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1_019e26c2-0007-7959-948c-3e842b9a0533" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.five9.com/role/StockholdersEquityScheduleofRSUActivityDetails" xlink:type="simple" xlink:href="fivn-20260630.xsd#StockholdersEquityScheduleofRSUActivityDetails"/>
  <link:definitionLink xlink:role="http://www.five9.com/role/StockholdersEquityScheduleofRSUActivityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e26c2-0007-7480-8704-367e425c5c97" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e26c2-0007-7f70-8f86-74ce25a41ae7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e26c2-0007-7480-8704-367e425c5c97" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e26c2-0007-7f70-8f86-74ce25a41ae7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019e26c2-0007-713e-b547-3cd131c61dc7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e26c2-0007-7f70-8f86-74ce25a41ae7" xlink:to="loc_us-gaap_AwardTypeAxis_019e26c2-0007-713e-b547-3cd131c61dc7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e26c2-0007-713e-b547-3cd131c61dc7_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019e26c2-0007-713e-b547-3cd131c61dc7" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e26c2-0007-713e-b547-3cd131c61dc7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e26c2-0007-7203-9f53-a59fad4c7332" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019e26c2-0007-713e-b547-3cd131c61dc7" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e26c2-0007-7203-9f53-a59fad4c7332" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_019e26c2-0007-7bc3-915d-89c4432eb087" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e26c2-0007-7203-9f53-a59fad4c7332" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_019e26c2-0007-7bc3-915d-89c4432eb087" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_PerformanceBasedAndMarketBasedRestrictedStockUnitsPRSUsMember_019e26c2-0007-70a0-a428-3ba02c593391" xlink:href="fivn-20260630.xsd#fivn_PerformanceBasedAndMarketBasedRestrictedStockUnitsPRSUsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e26c2-0007-7203-9f53-a59fad4c7332" xlink:to="loc_fivn_PerformanceBasedAndMarketBasedRestrictedStockUnitsPRSUsMember_019e26c2-0007-70a0-a428-3ba02c593391" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingRollForward_019e26c2-0007-78bc-a2e9-0433aa0fe2cc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e26c2-0007-7480-8704-367e425c5c97" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingRollForward_019e26c2-0007-78bc-a2e9-0433aa0fe2cc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019e26c2-0007-7350-8630-eb9583a8e3fd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingRollForward_019e26c2-0007-78bc-a2e9-0433aa0fe2cc" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019e26c2-0007-7350-8630-eb9583a8e3fd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019e26c2-0007-7a97-a9f7-a1e1bf9d8814" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingRollForward_019e26c2-0007-78bc-a2e9-0433aa0fe2cc" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019e26c2-0007-7a97-a9f7-a1e1bf9d8814" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_019e26c2-0007-761f-ae66-627dca9c9bab" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingRollForward_019e26c2-0007-78bc-a2e9-0433aa0fe2cc" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_019e26c2-0007-761f-ae66-627dca9c9bab" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_019e26c2-0007-7a6a-8525-dee3881305eb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingRollForward_019e26c2-0007-78bc-a2e9-0433aa0fe2cc" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_019e26c2-0007-7a6a-8525-dee3881305eb" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019e26c2-0007-7683-b366-bcf95fd45141" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e26c2-0007-7814-883f-cf6b70d0b431" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e26c2-0007-7480-8704-367e425c5c97" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e26c2-0007-7814-883f-cf6b70d0b431" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019e26c2-0007-7956-8fa6-aaedb242ccfb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e26c2-0007-7814-883f-cf6b70d0b431" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019e26c2-0007-7956-8fa6-aaedb242ccfb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019e26c2-0007-7f99-967c-da9fbc952c15" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e26c2-0007-7814-883f-cf6b70d0b431" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019e26c2-0007-7f99-967c-da9fbc952c15" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_019e26c2-0007-726d-8c53-6b237aa355b1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e26c2-0007-7814-883f-cf6b70d0b431" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_019e26c2-0007-726d-8c53-6b237aa355b1" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_019e26c2-0007-7915-a2ce-837be7b9a164" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e26c2-0007-7814-883f-cf6b70d0b431" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_019e26c2-0007-7915-a2ce-837be7b9a164" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019e26c2-0007-779b-866a-37f577d7bf50" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.five9.com/role/StockholdersEquityPerformanceBasedRestrictedStockUnitsNarrativeDetails" xlink:type="simple" xlink:href="fivn-20260630.xsd#StockholdersEquityPerformanceBasedRestrictedStockUnitsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.five9.com/role/StockholdersEquityPerformanceBasedRestrictedStockUnitsNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_fivn_StockholdersEquityCapitalStructureLineItems_019e26c2-0007-7709-a6cc-841930d29693" xlink:href="fivn-20260630.xsd#fivn_StockholdersEquityCapitalStructureLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_StockholdersEquityCapitalStructureTable_019e26c2-0007-705c-ab26-6b28a6b30a0e" xlink:href="fivn-20260630.xsd#fivn_StockholdersEquityCapitalStructureTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_fivn_StockholdersEquityCapitalStructureLineItems_019e26c2-0007-7709-a6cc-841930d29693" xlink:to="loc_fivn_StockholdersEquityCapitalStructureTable_019e26c2-0007-705c-ab26-6b28a6b30a0e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019e26c2-0007-77ad-899b-6e01a5055af2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_fivn_StockholdersEquityCapitalStructureTable_019e26c2-0007-705c-ab26-6b28a6b30a0e" xlink:to="loc_us-gaap_AwardTypeAxis_019e26c2-0007-77ad-899b-6e01a5055af2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e26c2-0007-77ad-899b-6e01a5055af2_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019e26c2-0007-77ad-899b-6e01a5055af2" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e26c2-0007-77ad-899b-6e01a5055af2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e26c2-0007-7c9e-b732-84468476bc32" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019e26c2-0007-77ad-899b-6e01a5055af2" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e26c2-0007-7c9e-b732-84468476bc32" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_MarketBasedPRSUsMember_019e26c2-0007-7099-a07a-096b2ad64227" xlink:href="fivn-20260630.xsd#fivn_MarketBasedPRSUsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e26c2-0007-7c9e-b732-84468476bc32" xlink:to="loc_fivn_MarketBasedPRSUsMember_019e26c2-0007-7099-a07a-096b2ad64227" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_PerformanceBasedRestrictedStockUnitsPRSUsMember_019e26c2-0007-7ee0-a7c2-d34a8811dd6d" xlink:href="fivn-20260630.xsd#fivn_PerformanceBasedRestrictedStockUnitsPRSUsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e26c2-0007-7c9e-b732-84468476bc32" xlink:to="loc_fivn_PerformanceBasedRestrictedStockUnitsPRSUsMember_019e26c2-0007-7ee0-a7c2-d34a8811dd6d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_PerformanceBasedRestrictedStockUnitsPRSUs20232025Member_019e26c2-0007-78fa-9838-61140efaa13f" xlink:href="fivn-20260630.xsd#fivn_PerformanceBasedRestrictedStockUnitsPRSUs20232025Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e26c2-0007-7c9e-b732-84468476bc32" xlink:to="loc_fivn_PerformanceBasedRestrictedStockUnitsPRSUs20232025Member_019e26c2-0007-78fa-9838-61140efaa13f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_PerformanceBasedRestrictedStockUnitsPRSUs20242026Member_019e26c2-0007-76b4-a325-b5309ba45e45" xlink:href="fivn-20260630.xsd#fivn_PerformanceBasedRestrictedStockUnitsPRSUs20242026Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e26c2-0007-7c9e-b732-84468476bc32" xlink:to="loc_fivn_PerformanceBasedRestrictedStockUnitsPRSUs20242026Member_019e26c2-0007-76b4-a325-b5309ba45e45" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualAxis_019e26c2-0007-7fe2-aebb-80953f7fc274" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_TitleOfIndividualAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_fivn_StockholdersEquityCapitalStructureTable_019e26c2-0007-705c-ab26-6b28a6b30a0e" xlink:to="loc_srt_TitleOfIndividualAxis_019e26c2-0007-7fe2-aebb-80953f7fc274" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019e26c2-0007-7fe2-aebb-80953f7fc274_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_TitleOfIndividualAxis_019e26c2-0007-7fe2-aebb-80953f7fc274" xlink:to="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019e26c2-0007-7fe2-aebb-80953f7fc274_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019e26c2-0007-752a-81f9-9d5672b7f472" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_TitleOfIndividualAxis_019e26c2-0007-7fe2-aebb-80953f7fc274" xlink:to="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019e26c2-0007-752a-81f9-9d5672b7f472" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ChiefExecutiveOfficerMember_019e26c2-0007-7e01-9b84-c5836fa39bda" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ChiefExecutiveOfficerMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019e26c2-0007-752a-81f9-9d5672b7f472" xlink:to="loc_srt_ChiefExecutiveOfficerMember_019e26c2-0007-7e01-9b84-c5836fa39bda" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_CertainExecutivesMember_019e26c2-0007-7172-bd58-5c752796ed6c" xlink:href="fivn-20260630.xsd#fivn_CertainExecutivesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019e26c2-0007-752a-81f9-9d5672b7f472" xlink:to="loc_fivn_CertainExecutivesMember_019e26c2-0007-7172-bd58-5c752796ed6c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019e26c2-0007-76f8-9e63-f8eeb93a150f" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_fivn_StockholdersEquityCapitalStructureTable_019e26c2-0007-705c-ab26-6b28a6b30a0e" xlink:to="loc_srt_RangeAxis_019e26c2-0007-76f8-9e63-f8eeb93a150f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e26c2-0007-76f8-9e63-f8eeb93a150f_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019e26c2-0007-76f8-9e63-f8eeb93a150f" xlink:to="loc_srt_RangeMember_019e26c2-0007-76f8-9e63-f8eeb93a150f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e26c2-0007-7620-8be8-a35a6a12fb87" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019e26c2-0007-76f8-9e63-f8eeb93a150f" xlink:to="loc_srt_RangeMember_019e26c2-0007-7620-8be8-a35a6a12fb87" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019e26c2-0007-7bf5-9bcd-2163e272830e" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e26c2-0007-7620-8be8-a35a6a12fb87" xlink:to="loc_srt_MinimumMember_019e26c2-0007-7bf5-9bcd-2163e272830e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019e26c2-0007-73cb-a1d0-3eac377122d3" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e26c2-0007-7620-8be8-a35a6a12fb87" xlink:to="loc_srt_MaximumMember_019e26c2-0007-73cb-a1d0-3eac377122d3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_EquityInstrumentConditionAxis_019e26c2-0007-7422-8462-572cfa0aee96" xlink:href="fivn-20260630.xsd#fivn_EquityInstrumentConditionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_fivn_StockholdersEquityCapitalStructureTable_019e26c2-0007-705c-ab26-6b28a6b30a0e" xlink:to="loc_fivn_EquityInstrumentConditionAxis_019e26c2-0007-7422-8462-572cfa0aee96" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_EquityInstrumentConditionDomain_019e26c2-0007-7422-8462-572cfa0aee96_default" xlink:href="fivn-20260630.xsd#fivn_EquityInstrumentConditionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_fivn_EquityInstrumentConditionAxis_019e26c2-0007-7422-8462-572cfa0aee96" xlink:to="loc_fivn_EquityInstrumentConditionDomain_019e26c2-0007-7422-8462-572cfa0aee96_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_EquityInstrumentConditionDomain_019e26c2-0007-7524-9013-1eff0b5960c4" xlink:href="fivn-20260630.xsd#fivn_EquityInstrumentConditionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_fivn_EquityInstrumentConditionAxis_019e26c2-0007-7422-8462-572cfa0aee96" xlink:to="loc_fivn_EquityInstrumentConditionDomain_019e26c2-0007-7524-9013-1eff0b5960c4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_ShareholderForPerformanceAnyPeriodIsNegativeMember_019e26c2-0007-7108-b972-45e8bf6571b5" xlink:href="fivn-20260630.xsd#fivn_ShareholderForPerformanceAnyPeriodIsNegativeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fivn_EquityInstrumentConditionDomain_019e26c2-0007-7524-9013-1eff0b5960c4" xlink:to="loc_fivn_ShareholderForPerformanceAnyPeriodIsNegativeMember_019e26c2-0007-7108-b972-45e8bf6571b5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_ExecutiveEmploymentTerminatesDueToDeathByEndOf2024Member_019e26c2-0007-7e98-b2da-143cd7aca881" xlink:href="fivn-20260630.xsd#fivn_ExecutiveEmploymentTerminatesDueToDeathByEndOf2024Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fivn_EquityInstrumentConditionDomain_019e26c2-0007-7524-9013-1eff0b5960c4" xlink:to="loc_fivn_ExecutiveEmploymentTerminatesDueToDeathByEndOf2024Member_019e26c2-0007-7e98-b2da-143cd7aca881" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_ExecutiveEmploymentTerminatesDueToDisabilityByEndOf2024Member_019e26c2-0007-7a62-97bb-0bd4dfbb6158" xlink:href="fivn-20260630.xsd#fivn_ExecutiveEmploymentTerminatesDueToDisabilityByEndOf2024Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fivn_EquityInstrumentConditionDomain_019e26c2-0007-7524-9013-1eff0b5960c4" xlink:to="loc_fivn_ExecutiveEmploymentTerminatesDueToDisabilityByEndOf2024Member_019e26c2-0007-7a62-97bb-0bd4dfbb6158" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019e26c2-0007-7904-bf9c-457ad36321a1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fivn_StockholdersEquityCapitalStructureLineItems_019e26c2-0007-7709-a6cc-841930d29693" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019e26c2-0007-7904-bf9c-457ad36321a1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThenOptionsWeightedAverageGrantDateFairValue_019e26c2-0007-745f-a8d6-07709559d1dd" xlink:href="fivn-20260630.xsd#fivn_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThenOptionsWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fivn_StockholdersEquityCapitalStructureLineItems_019e26c2-0007-7709-a6cc-841930d29693" xlink:to="loc_fivn_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThenOptionsWeightedAverageGrantDateFairValue_019e26c2-0007-745f-a8d6-07709559d1dd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentOfTargetedAward_019e26c2-0007-77c9-8868-44506a500b82" xlink:href="fivn-20260630.xsd#fivn_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentOfTargetedAward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fivn_StockholdersEquityCapitalStructureLineItems_019e26c2-0007-7709-a6cc-841930d29693" xlink:to="loc_fivn_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentOfTargetedAward_019e26c2-0007-77c9-8868-44506a500b82" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfOverlappingMeasurementPeriods_019e26c2-0007-76e6-97bf-275533d09dab" xlink:href="fivn-20260630.xsd#fivn_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfOverlappingMeasurementPeriods"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fivn_StockholdersEquityCapitalStructureLineItems_019e26c2-0007-7709-a6cc-841930d29693" xlink:to="loc_fivn_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfOverlappingMeasurementPeriods_019e26c2-0007-76e6-97bf-275533d09dab" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfOverlappingMeasurementTermOne_019e26c2-0007-70a4-9da2-98e10cd20618" xlink:href="fivn-20260630.xsd#fivn_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfOverlappingMeasurementTermOne"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fivn_StockholdersEquityCapitalStructureLineItems_019e26c2-0007-7709-a6cc-841930d29693" xlink:to="loc_fivn_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfOverlappingMeasurementTermOne_019e26c2-0007-70a4-9da2-98e10cd20618" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfOverlappingMeasurementTermTwo_019e26c2-0007-7ae4-b778-93bbf8442b34" xlink:href="fivn-20260630.xsd#fivn_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfOverlappingMeasurementTermTwo"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fivn_StockholdersEquityCapitalStructureLineItems_019e26c2-0007-7709-a6cc-841930d29693" xlink:to="loc_fivn_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfOverlappingMeasurementTermTwo_019e26c2-0007-7ae4-b778-93bbf8442b34" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentOfAwardOverlappingMeasurementPeriodOne_019e26c2-0007-70c9-8f7c-48541267eb8b" xlink:href="fivn-20260630.xsd#fivn_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentOfAwardOverlappingMeasurementPeriodOne"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fivn_StockholdersEquityCapitalStructureLineItems_019e26c2-0007-7709-a6cc-841930d29693" xlink:to="loc_fivn_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentOfAwardOverlappingMeasurementPeriodOne_019e26c2-0007-70c9-8f7c-48541267eb8b" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentOfAwardOverlappingMeasurementPeriodTwo_019e26c2-0007-77b1-bebe-a65402187bfd" xlink:href="fivn-20260630.xsd#fivn_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentOfAwardOverlappingMeasurementPeriodTwo"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fivn_StockholdersEquityCapitalStructureLineItems_019e26c2-0007-7709-a6cc-841930d29693" xlink:to="loc_fivn_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentOfAwardOverlappingMeasurementPeriodTwo_019e26c2-0007-77b1-bebe-a65402187bfd" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumPercentOfAward_019e26c2-0007-7827-9a4e-2e7a52a920d6" xlink:href="fivn-20260630.xsd#fivn_ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumPercentOfAward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fivn_StockholdersEquityCapitalStructureLineItems_019e26c2-0007-7709-a6cc-841930d29693" xlink:to="loc_fivn_ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumPercentOfAward_019e26c2-0007-7827-9a4e-2e7a52a920d6" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_ShareBasedCompensationArrangementByShareBasedPaymentAwardWeightedPercentageOfSubscriptionRevenueGrowthRate_019e26c2-0007-7e67-899d-86d90e248c53" xlink:href="fivn-20260630.xsd#fivn_ShareBasedCompensationArrangementByShareBasedPaymentAwardWeightedPercentageOfSubscriptionRevenueGrowthRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fivn_StockholdersEquityCapitalStructureLineItems_019e26c2-0007-7709-a6cc-841930d29693" xlink:to="loc_fivn_ShareBasedCompensationArrangementByShareBasedPaymentAwardWeightedPercentageOfSubscriptionRevenueGrowthRate_019e26c2-0007-7e67-899d-86d90e248c53" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_ShareBasedCompensationArrangementByShareBasedPaymentAwardWeightedPercentageOfRelativeTotalShareholderReturn_019e26c2-0007-7360-8773-125e8a41755a" xlink:href="fivn-20260630.xsd#fivn_ShareBasedCompensationArrangementByShareBasedPaymentAwardWeightedPercentageOfRelativeTotalShareholderReturn"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fivn_StockholdersEquityCapitalStructureLineItems_019e26c2-0007-7709-a6cc-841930d29693" xlink:to="loc_fivn_ShareBasedCompensationArrangementByShareBasedPaymentAwardWeightedPercentageOfRelativeTotalShareholderReturn_019e26c2-0007-7360-8773-125e8a41755a" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentOfTotalShareholderReturn_019e26c2-0007-7c51-82f3-d1065f53d800" xlink:href="fivn-20260630.xsd#fivn_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentOfTotalShareholderReturn"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fivn_StockholdersEquityCapitalStructureLineItems_019e26c2-0007-7709-a6cc-841930d29693" xlink:to="loc_fivn_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentOfTotalShareholderReturn_019e26c2-0007-7c51-82f3-d1065f53d800" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentOfRelativeTotalShareholderReturnRanking_019e26c2-0007-7830-b93a-c9f7ea48e49d" xlink:href="fivn-20260630.xsd#fivn_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentOfRelativeTotalShareholderReturnRanking"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fivn_StockholdersEquityCapitalStructureLineItems_019e26c2-0007-7709-a6cc-841930d29693" xlink:to="loc_fivn_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentOfRelativeTotalShareholderReturnRanking_019e26c2-0007-7830-b93a-c9f7ea48e49d" xlink:type="arc" order="12"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.five9.com/role/StockholdersEquityStockbasedCompensationDetails" xlink:type="simple" xlink:href="fivn-20260630.xsd#StockholdersEquityStockbasedCompensationDetails"/>
  <link:definitionLink xlink:role="http://www.five9.com/role/StockholdersEquityStockbasedCompensationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_019e26c2-0007-7cfd-b15b-6eeb6619333c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_019e26c2-0007-73b5-b5ca-7280a89a6f96" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_019e26c2-0007-7cfd-b15b-6eeb6619333c" xlink:to="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_019e26c2-0007-73b5-b5ca-7280a89a6f96" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementOfIncomeLocationBalanceAxis_019e26c2-0007-744b-bcc2-6d7671fc42e4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementOfIncomeLocationBalanceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_019e26c2-0007-73b5-b5ca-7280a89a6f96" xlink:to="loc_us-gaap_StatementOfIncomeLocationBalanceAxis_019e26c2-0007-744b-bcc2-6d7671fc42e4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019e26c2-0007-77e7-b176-e2a1d170982b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_019e26c2-0007-73b5-b5ca-7280a89a6f96" xlink:to="loc_us-gaap_AwardTypeAxis_019e26c2-0007-77e7-b176-e2a1d170982b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e26c2-0007-77e7-b176-e2a1d170982b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019e26c2-0007-77e7-b176-e2a1d170982b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e26c2-0007-77e7-b176-e2a1d170982b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e26c2-0007-78e7-850a-144f52f5fd49" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019e26c2-0007-77e7-b176-e2a1d170982b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e26c2-0007-78e7-850a-144f52f5fd49" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_019e26c2-0007-7590-8a00-77e1e9bdd635" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e26c2-0007-78e7-850a-144f52f5fd49" xlink:to="loc_us-gaap_EmployeeStockOptionMember_019e26c2-0007-7590-8a00-77e1e9bdd635" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_019e26c2-0007-7974-9307-32823acbc51c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e26c2-0007-78e7-850a-144f52f5fd49" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_019e26c2-0007-7974-9307-32823acbc51c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_PerformanceBasedRestrictedStockUnitsMember_019e26c2-0007-7a6b-bda0-21a8e7d9b8b8" xlink:href="fivn-20260630.xsd#fivn_PerformanceBasedRestrictedStockUnitsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e26c2-0007-78e7-850a-144f52f5fd49" xlink:to="loc_fivn_PerformanceBasedRestrictedStockUnitsMember_019e26c2-0007-7a6b-bda0-21a8e7d9b8b8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember_019e26c2-0007-7034-8572-f14042082b2c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e26c2-0007-78e7-850a-144f52f5fd49" xlink:to="loc_us-gaap_EmployeeStockMember_019e26c2-0007-7034-8572-f14042082b2c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_019e26c2-0007-7eec-bdbe-4ba03c7913f8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_019e26c2-0007-7cfd-b15b-6eeb6619333c" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_019e26c2-0007-7eec-bdbe-4ba03c7913f8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_019e26c2-0007-7c86-b4f5-586ee761baa5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_019e26c2-0007-7cfd-b15b-6eeb6619333c" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_019e26c2-0007-7c86-b4f5-586ee761baa5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_019e26c2-0007-715a-bf49-49023c92a693" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_019e26c2-0007-7cfd-b15b-6eeb6619333c" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_019e26c2-0007-715a-bf49-49023c92a693" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.five9.com/role/StockholdersEquityStockbasedCompensationDetails_1" xlink:type="simple" xlink:href="fivn-20260630.xsd#StockholdersEquityStockbasedCompensationDetails_1"/>
  <link:definitionLink xlink:role="http://www.five9.com/role/StockholdersEquityStockbasedCompensationDetails_1" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_019e26c2-0007-7cfd-b15b-6eeb6619333c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_019e26c2-0007-73b5-b5ca-7280a89a6f96" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_019e26c2-0007-7cfd-b15b-6eeb6619333c" xlink:to="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_019e26c2-0007-73b5-b5ca-7280a89a6f96" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019e26c2-0007-77e7-b176-e2a1d170982b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_019e26c2-0007-73b5-b5ca-7280a89a6f96" xlink:to="loc_us-gaap_AwardTypeAxis_019e26c2-0007-77e7-b176-e2a1d170982b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e26c2-0007-77e7-b176-e2a1d170982b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019e26c2-0007-77e7-b176-e2a1d170982b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e26c2-0007-77e7-b176-e2a1d170982b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e26c2-0007-78e7-850a-144f52f5fd49" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019e26c2-0007-77e7-b176-e2a1d170982b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e26c2-0007-78e7-850a-144f52f5fd49" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_019e26c2-0007-7590-8a00-77e1e9bdd635" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e26c2-0007-78e7-850a-144f52f5fd49" xlink:to="loc_us-gaap_EmployeeStockOptionMember_019e26c2-0007-7590-8a00-77e1e9bdd635" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_019e26c2-0007-7974-9307-32823acbc51c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e26c2-0007-78e7-850a-144f52f5fd49" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_019e26c2-0007-7974-9307-32823acbc51c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_PerformanceBasedRestrictedStockUnitsMember_019e26c2-0007-7a6b-bda0-21a8e7d9b8b8" xlink:href="fivn-20260630.xsd#fivn_PerformanceBasedRestrictedStockUnitsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e26c2-0007-78e7-850a-144f52f5fd49" xlink:to="loc_fivn_PerformanceBasedRestrictedStockUnitsMember_019e26c2-0007-7a6b-bda0-21a8e7d9b8b8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember_019e26c2-0007-7034-8572-f14042082b2c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e26c2-0007-78e7-850a-144f52f5fd49" xlink:to="loc_us-gaap_EmployeeStockMember_019e26c2-0007-7034-8572-f14042082b2c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_019e26c2-0007-7eec-bdbe-4ba03c7913f8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_019e26c2-0007-7cfd-b15b-6eeb6619333c" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_019e26c2-0007-7eec-bdbe-4ba03c7913f8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_019e26c2-0007-7c86-b4f5-586ee761baa5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_019e26c2-0007-7cfd-b15b-6eeb6619333c" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_019e26c2-0007-7c86-b4f5-586ee761baa5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_019e26c2-0007-715a-bf49-49023c92a693" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_019e26c2-0007-7cfd-b15b-6eeb6619333c" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_019e26c2-0007-715a-bf49-49023c92a693" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.five9.com/role/StockholdersEquityValuationAssumptionsDetails" xlink:type="simple" xlink:href="fivn-20260630.xsd#StockholdersEquityValuationAssumptionsDetails"/>
  <link:definitionLink xlink:role="http://www.five9.com/role/StockholdersEquityValuationAssumptionsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockLineItems_019e26c2-0007-7e3f-a202-dc67bc85039c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_019e26c2-0007-7bf4-a8ba-7362c6500e31" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfStockByClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_019e26c2-0007-7e3f-a202-dc67bc85039c" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_019e26c2-0007-7bf4-a8ba-7362c6500e31" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019e26c2-0007-759c-a1eb-fc44fea628f3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_019e26c2-0007-7bf4-a8ba-7362c6500e31" xlink:to="loc_us-gaap_AwardTypeAxis_019e26c2-0007-759c-a1eb-fc44fea628f3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e26c2-0007-759c-a1eb-fc44fea628f3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019e26c2-0007-759c-a1eb-fc44fea628f3" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e26c2-0007-759c-a1eb-fc44fea628f3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e26c2-0007-7134-9571-81116e46d4e1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019e26c2-0007-759c-a1eb-fc44fea628f3" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e26c2-0007-7134-9571-81116e46d4e1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_PerformanceBasedRestrictedStockUnitsPRSUsMember_019e26c2-0007-7633-96c5-e237e40b48da" xlink:href="fivn-20260630.xsd#fivn_PerformanceBasedRestrictedStockUnitsPRSUsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e26c2-0007-7134-9571-81116e46d4e1" xlink:to="loc_fivn_PerformanceBasedRestrictedStockUnitsPRSUsMember_019e26c2-0007-7633-96c5-e237e40b48da" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_019e26c2-0007-79cb-9550-1901d4cab778" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019e26c2-0007-7e3f-a202-dc67bc85039c" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_019e26c2-0007-79cb-9550-1901d4cab778" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_019e26c2-0007-7eb4-b291-94d3219820e9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019e26c2-0007-7e3f-a202-dc67bc85039c" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_019e26c2-0007-7eb4-b291-94d3219820e9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_019e26c2-0007-7247-b48c-f8d1f98f3304" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019e26c2-0007-7e3f-a202-dc67bc85039c" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_019e26c2-0007-7247-b48c-f8d1f98f3304" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_019e26c2-0007-775b-ad8e-24d98205ee4e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_019e26c2-0007-7e3f-a202-dc67bc85039c" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_019e26c2-0007-775b-ad8e-24d98205ee4e" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.five9.com/role/CommitmentsandContingenciesDetails" xlink:type="simple" xlink:href="fivn-20260630.xsd#CommitmentsandContingenciesDetails"/>
  <link:definitionLink xlink:role="http://www.five9.com/role/CommitmentsandContingenciesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCommitmentsLineItems_019e26c2-0007-7e1c-8e4b-5cbfc77eeccd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherCommitmentsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCommitmentsTable_019e26c2-0007-7fcd-8b48-41cbce896066" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherCommitmentsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019e26c2-0007-7e1c-8e4b-5cbfc77eeccd" xlink:to="loc_us-gaap_OtherCommitmentsTable_019e26c2-0007-7fcd-8b48-41cbce896066" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCommitmentsAxis_019e26c2-0007-7bba-9042-3f3325ae0367" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherCommitmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_OtherCommitmentsTable_019e26c2-0007-7fcd-8b48-41cbce896066" xlink:to="loc_us-gaap_OtherCommitmentsAxis_019e26c2-0007-7bba-9042-3f3325ae0367" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCommitmentsDomain_019e26c2-0007-7bba-9042-3f3325ae0367_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherCommitmentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_OtherCommitmentsAxis_019e26c2-0007-7bba-9042-3f3325ae0367" xlink:to="loc_us-gaap_OtherCommitmentsDomain_019e26c2-0007-7bba-9042-3f3325ae0367_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCommitmentsDomain_019e26c2-0007-7011-9bc5-f8124c165710" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherCommitmentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_OtherCommitmentsAxis_019e26c2-0007-7bba-9042-3f3325ae0367" xlink:to="loc_us-gaap_OtherCommitmentsDomain_019e26c2-0007-7011-9bc5-f8124c165710" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_HostingandTelecommunicationsAgreementMember_019e26c2-0007-7203-ab72-b7987509a2c0" xlink:href="fivn-20260630.xsd#fivn_HostingandTelecommunicationsAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsDomain_019e26c2-0007-7011-9bc5-f8124c165710" xlink:to="loc_fivn_HostingandTelecommunicationsAgreementMember_019e26c2-0007-7203-ab72-b7987509a2c0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_PurchaseCommitmentCloudServicesAndSoftwareAndMaintenanceAgreementMember_019e26c2-0007-7186-8df5-5650cd52be56" xlink:href="fivn-20260630.xsd#fivn_PurchaseCommitmentCloudServicesAndSoftwareAndMaintenanceAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsDomain_019e26c2-0007-7011-9bc5-f8124c165710" xlink:to="loc_fivn_PurchaseCommitmentCloudServicesAndSoftwareAndMaintenanceAgreementMember_019e26c2-0007-7186-8df5-5650cd52be56" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_ResellerAgreementMember_019e26c2-0007-7fdd-85d6-4ea57125923c" xlink:href="fivn-20260630.xsd#fivn_ResellerAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsDomain_019e26c2-0007-7011-9bc5-f8124c165710" xlink:to="loc_fivn_ResellerAgreementMember_019e26c2-0007-7fdd-85d6-4ea57125923c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019e26c2-0007-7850-adc2-c75c4cefa923" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_OtherCommitmentsTable_019e26c2-0007-7fcd-8b48-41cbce896066" xlink:to="loc_srt_RangeAxis_019e26c2-0007-7850-adc2-c75c4cefa923" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e26c2-0007-7850-adc2-c75c4cefa923_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019e26c2-0007-7850-adc2-c75c4cefa923" xlink:to="loc_srt_RangeMember_019e26c2-0007-7850-adc2-c75c4cefa923_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e26c2-0007-7c32-bd15-3230f8e6c4a8" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019e26c2-0007-7850-adc2-c75c4cefa923" xlink:to="loc_srt_RangeMember_019e26c2-0007-7c32-bd15-3230f8e6c4a8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019e26c2-0007-7524-bd6d-ae885ef7c6da" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e26c2-0007-7c32-bd15-3230f8e6c4a8" xlink:to="loc_srt_MaximumMember_019e26c2-0007-7524-bd6d-ae885ef7c6da" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019e26c2-0007-73b4-a1ef-223ba0635f00" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_OtherCommitmentsTable_019e26c2-0007-7fcd-8b48-41cbce896066" xlink:to="loc_us-gaap_DebtInstrumentAxis_019e26c2-0007-73b4-a1ef-223ba0635f00" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e26c2-0007-73b4-a1ef-223ba0635f00_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e26c2-0007-73b4-a1ef-223ba0635f00" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e26c2-0007-73b4-a1ef-223ba0635f00_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019e26c2-0007-7f12-b2d2-d2dce4f66f28" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019e26c2-0007-73b4-a1ef-223ba0635f00" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019e26c2-0007-7f12-b2d2-d2dce4f66f28" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_ConvertibleSeniorNotesDue2029Member_019e26c2-0007-755c-9d61-72926cc50128" xlink:href="fivn-20260630.xsd#fivn_ConvertibleSeniorNotesDue2029Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019e26c2-0007-7f12-b2d2-d2dce4f66f28" xlink:to="loc_fivn_ConvertibleSeniorNotesDue2029Member_019e26c2-0007-755c-9d61-72926cc50128" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019e26c2-0007-70ac-a290-ee4605bf0808" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_OtherCommitmentsTable_019e26c2-0007-7fcd-8b48-41cbce896066" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019e26c2-0007-70ac-a290-ee4605bf0808" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e26c2-0007-70ac-a290-ee4605bf0808_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e26c2-0007-70ac-a290-ee4605bf0808" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e26c2-0007-70ac-a290-ee4605bf0808_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019e26c2-0007-720e-b65f-b477566712a6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019e26c2-0007-70ac-a290-ee4605bf0808" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019e26c2-0007-720e-b65f-b477566712a6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtMember_019e26c2-0007-7b19-b865-bb6818f2bcfc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConvertibleDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019e26c2-0007-720e-b65f-b477566712a6" xlink:to="loc_us-gaap_ConvertibleDebtMember_019e26c2-0007-7b19-b865-bb6818f2bcfc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCommitment_019e26c2-0007-712d-8319-da40a64ccf5c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherCommitment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019e26c2-0007-7e1c-8e4b-5cbfc77eeccd" xlink:to="loc_us-gaap_OtherCommitment_019e26c2-0007-712d-8319-da40a64ccf5c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_OtherCommitmentFutureMinimumPaymentsDuePeriod_019e26c2-0007-75e9-9e40-fb649c839f46" xlink:href="fivn-20260630.xsd#fivn_OtherCommitmentFutureMinimumPaymentsDuePeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019e26c2-0007-7e1c-8e4b-5cbfc77eeccd" xlink:to="loc_fivn_OtherCommitmentFutureMinimumPaymentsDuePeriod_019e26c2-0007-75e9-9e40-fb649c839f46" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_CommercialCommitmentsExpiring_019e26c2-0007-7d41-8788-ba6a6cfd2c8a" xlink:href="fivn-20260630.xsd#fivn_CommercialCommitmentsExpiring"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019e26c2-0007-7e1c-8e4b-5cbfc77eeccd" xlink:to="loc_fivn_CommercialCommitmentsExpiring_019e26c2-0007-7d41-8788-ba6a6cfd2c8a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_CommercialCommitmentToBePaidRemainderOfTheYear_019e26c2-0007-7f0f-9cc3-7cfb0222cd21" xlink:href="fivn-20260630.xsd#fivn_CommercialCommitmentToBePaidRemainderOfTheYear"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019e26c2-0007-7e1c-8e4b-5cbfc77eeccd" xlink:to="loc_fivn_CommercialCommitmentToBePaidRemainderOfTheYear_019e26c2-0007-7f0f-9cc3-7cfb0222cd21" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_CommercialCommitmentToBePaidYearOne_019e26c2-0007-7c8f-90dd-9d1d5ad05674" xlink:href="fivn-20260630.xsd#fivn_CommercialCommitmentToBePaidYearOne"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019e26c2-0007-7e1c-8e4b-5cbfc77eeccd" xlink:to="loc_fivn_CommercialCommitmentToBePaidYearOne_019e26c2-0007-7c8f-90dd-9d1d5ad05674" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_CommercialCommitmentToBePaidYearTwo_019e26c2-0007-7910-b68d-c61b0e0fa71e" xlink:href="fivn-20260630.xsd#fivn_CommercialCommitmentToBePaidYearTwo"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019e26c2-0007-7e1c-8e4b-5cbfc77eeccd" xlink:to="loc_fivn_CommercialCommitmentToBePaidYearTwo_019e26c2-0007-7910-b68d-c61b0e0fa71e" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_CommercialCommitmentToBePaidYearThree_019e26c2-0007-7f15-adc2-5e0a51040727" xlink:href="fivn-20260630.xsd#fivn_CommercialCommitmentToBePaidYearThree"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019e26c2-0007-7e1c-8e4b-5cbfc77eeccd" xlink:to="loc_fivn_CommercialCommitmentToBePaidYearThree_019e26c2-0007-7f15-adc2-5e0a51040727" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermPurchaseCommitmentAmount_019e26c2-0007-7cac-9a3d-763674b08e7f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermPurchaseCommitmentAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019e26c2-0007-7e1c-8e4b-5cbfc77eeccd" xlink:to="loc_us-gaap_LongTermPurchaseCommitmentAmount_019e26c2-0007-7cac-9a3d-763674b08e7f" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PurchaseCommitmentRemainingMinimumAmountCommitted_019e26c2-0007-77e4-9c7b-df0eb0b5088c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PurchaseCommitmentRemainingMinimumAmountCommitted"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019e26c2-0007-7e1c-8e4b-5cbfc77eeccd" xlink:to="loc_us-gaap_PurchaseCommitmentRemainingMinimumAmountCommitted_019e26c2-0007-77e4-9c7b-df0eb0b5088c" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_FinancingArrangementCommitmentAmount_019e26c2-0007-704d-a389-a170c564030a" xlink:href="fivn-20260630.xsd#fivn_FinancingArrangementCommitmentAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019e26c2-0007-7e1c-8e4b-5cbfc77eeccd" xlink:to="loc_fivn_FinancingArrangementCommitmentAmount_019e26c2-0007-704d-a389-a170c564030a" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_FinancingArrangementTerm_019e26c2-0007-7bdf-aecc-bd3940b9c3bf" xlink:href="fivn-20260630.xsd#fivn_FinancingArrangementTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019e26c2-0007-7e1c-8e4b-5cbfc77eeccd" xlink:to="loc_fivn_FinancingArrangementTerm_019e26c2-0007-7bdf-aecc-bd3940b9c3bf" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_FinancingArrangementNetPresentValue_019e26c2-0007-7ef6-92d6-f1da6a15d98d" xlink:href="fivn-20260630.xsd#fivn_FinancingArrangementNetPresentValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019e26c2-0007-7e1c-8e4b-5cbfc77eeccd" xlink:to="loc_fivn_FinancingArrangementNetPresentValue_019e26c2-0007-7ef6-92d6-f1da6a15d98d" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_FinancingArrangementNumberOfObligatedInstallmentPayments_019e26c2-0007-7016-a3cc-94bf3fd01a59" xlink:href="fivn-20260630.xsd#fivn_FinancingArrangementNumberOfObligatedInstallmentPayments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019e26c2-0007-7e1c-8e4b-5cbfc77eeccd" xlink:to="loc_fivn_FinancingArrangementNumberOfObligatedInstallmentPayments_019e26c2-0007-7016-a3cc-94bf3fd01a59" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_FinancingArrangementAnnualObligatedInstallmentPayments_019e26c2-0007-7d0b-8afc-252bab7c934f" xlink:href="fivn-20260630.xsd#fivn_FinancingArrangementAnnualObligatedInstallmentPayments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019e26c2-0007-7e1c-8e4b-5cbfc77eeccd" xlink:to="loc_fivn_FinancingArrangementAnnualObligatedInstallmentPayments_019e26c2-0007-7d0b-8afc-252bab7c934f" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_FinancingArrangementNumberOfRemainingObligatedAnnualPayments_019e26c2-0007-71db-a2f5-d6d3835e3410" xlink:href="fivn-20260630.xsd#fivn_FinancingArrangementNumberOfRemainingObligatedAnnualPayments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019e26c2-0007-7e1c-8e4b-5cbfc77eeccd" xlink:to="loc_fivn_FinancingArrangementNumberOfRemainingObligatedAnnualPayments_019e26c2-0007-71db-a2f5-d6d3835e3410" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_FinancingLiabilityCurrent_019e26c2-0007-734d-91cf-373cf0f41c4a" xlink:href="fivn-20260630.xsd#fivn_FinancingLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019e26c2-0007-7e1c-8e4b-5cbfc77eeccd" xlink:to="loc_fivn_FinancingLiabilityCurrent_019e26c2-0007-734d-91cf-373cf0f41c4a" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_FinancingLiabilityNoncurrent_019e26c2-0007-7bca-a641-233888f09463" xlink:href="fivn-20260630.xsd#fivn_FinancingLiabilityNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019e26c2-0007-7e1c-8e4b-5cbfc77eeccd" xlink:to="loc_fivn_FinancingLiabilityNoncurrent_019e26c2-0007-7bca-a641-233888f09463" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentCarryingAmount_019e26c2-0007-794b-8860-77b42b27bf3d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentCarryingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_019e26c2-0007-7e1c-8e4b-5cbfc77eeccd" xlink:to="loc_us-gaap_DebtInstrumentCarryingAmount_019e26c2-0007-794b-8860-77b42b27bf3d" xlink:type="arc" order="17"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.five9.com/role/SegmentandGeographicalInformationScheduleofRevenueandPropertyandEquipmentbyGeographicRegionDetails" xlink:type="simple" xlink:href="fivn-20260630.xsd#SegmentandGeographicalInformationScheduleofRevenueandPropertyandEquipmentbyGeographicRegionDetails"/>
  <link:definitionLink xlink:role="http://www.five9.com/role/SegmentandGeographicalInformationScheduleofRevenueandPropertyandEquipmentbyGeographicRegionDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_019e26c2-0007-7339-8480-20feb7740a9a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable_019e26c2-0007-7150-8566-29c15ca0727d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_019e26c2-0007-7339-8480-20feb7740a9a" xlink:to="loc_us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable_019e26c2-0007-7150-8566-29c15ca0727d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_019e26c2-0007-79a1-b1d0-cc7ba0006959" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable_019e26c2-0007-7150-8566-29c15ca0727d" xlink:to="loc_srt_StatementGeographicalAxis_019e26c2-0007-79a1-b1d0-cc7ba0006959" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_019e26c2-0007-79a1-b1d0-cc7ba0006959_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_019e26c2-0007-79a1-b1d0-cc7ba0006959" xlink:to="loc_srt_SegmentGeographicalDomain_019e26c2-0007-79a1-b1d0-cc7ba0006959_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_019e26c2-0007-79c4-a8d8-cdf7d2959cb4" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_019e26c2-0007-79a1-b1d0-cc7ba0006959" xlink:to="loc_srt_SegmentGeographicalDomain_019e26c2-0007-79c4-a8d8-cdf7d2959cb4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_019e26c2-0007-78ed-a534-01e55f999c6f" xlink:href="https://xbrl.sec.gov/country/2026/country-2026.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019e26c2-0007-79c4-a8d8-cdf7d2959cb4" xlink:to="loc_country_US_019e26c2-0007-78ed-a534-01e55f999c6f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NonUsMember_019e26c2-0007-7659-908e-c59b1f785154" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NonUsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019e26c2-0007-79c4-a8d8-cdf7d2959cb4" xlink:to="loc_us-gaap_NonUsMember_019e26c2-0007-7659-908e-c59b1f785154" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019e26c2-0007-7eca-a782-224a9da0318b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_019e26c2-0007-7339-8480-20feb7740a9a" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019e26c2-0007-7eca-a782-224a9da0318b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization_019e26c2-0007-7548-a6f2-5ed6b3589160" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_019e26c2-0007-7339-8480-20feb7740a9a" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentAndFinanceLeaseRightOfUseAssetAfterAccumulatedDepreciationAndAmortization_019e26c2-0007-7548-a6f2-5ed6b3589160" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.five9.com/role/SegmentandGeographicalInformationReconciliationofRevenuefromSegmentstoConsolidatedDetails" xlink:type="simple" xlink:href="fivn-20260630.xsd#SegmentandGeographicalInformationReconciliationofRevenuefromSegmentstoConsolidatedDetails"/>
  <link:definitionLink xlink:role="http://www.five9.com/role/SegmentandGeographicalInformationReconciliationofRevenuefromSegmentstoConsolidatedDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_019e26c2-0007-74e2-9e5b-845605ba36f8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e26c2-0007-7b33-8eef-d814bb65ec96" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e26c2-0007-74e2-9e5b-845605ba36f8" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e26c2-0007-7b33-8eef-d814bb65ec96" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019e26c2-0007-716a-808c-84ea1aab27c9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019e26c2-0007-7b33-8eef-d814bb65ec96" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019e26c2-0007-716a-808c-84ea1aab27c9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019e26c2-0007-716a-808c-84ea1aab27c9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019e26c2-0007-716a-808c-84ea1aab27c9" xlink:to="loc_us-gaap_SegmentDomain_019e26c2-0007-716a-808c-84ea1aab27c9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019e26c2-0007-7e5d-a058-e477e1b1cfbb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019e26c2-0007-716a-808c-84ea1aab27c9" xlink:to="loc_us-gaap_SegmentDomain_019e26c2-0007-7e5d-a058-e477e1b1cfbb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_ReportableSegmentMember_019e26c2-0007-7853-8317-52496bcd9e4a" xlink:href="fivn-20260630.xsd#fivn_ReportableSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019e26c2-0007-7e5d-a058-e477e1b1cfbb" xlink:to="loc_fivn_ReportableSegmentMember_019e26c2-0007-7853-8317-52496bcd9e4a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019e26c2-0007-72cd-820a-d1f596936574" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e26c2-0007-74e2-9e5b-845605ba36f8" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019e26c2-0007-72cd-820a-d1f596936574" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_CostOfRevenueAdjusted_019e26c2-0007-7851-9949-d6143c92c097" xlink:href="fivn-20260630.xsd#fivn_CostOfRevenueAdjusted"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e26c2-0007-74e2-9e5b-845605ba36f8" xlink:to="loc_fivn_CostOfRevenueAdjusted_019e26c2-0007-7851-9949-d6143c92c097" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_ResearchAndDevelopmentExpenseAdjusted_019e26c2-0007-77a0-901e-5e2ef6a6477e" xlink:href="fivn-20260630.xsd#fivn_ResearchAndDevelopmentExpenseAdjusted"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e26c2-0007-74e2-9e5b-845605ba36f8" xlink:to="loc_fivn_ResearchAndDevelopmentExpenseAdjusted_019e26c2-0007-77a0-901e-5e2ef6a6477e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_AdjustedSalesAndMarketingExpense_019e26c2-0007-7ffa-ae54-fc8553a08c77" xlink:href="fivn-20260630.xsd#fivn_AdjustedSalesAndMarketingExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e26c2-0007-74e2-9e5b-845605ba36f8" xlink:to="loc_fivn_AdjustedSalesAndMarketingExpense_019e26c2-0007-7ffa-ae54-fc8553a08c77" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_GeneralAndAdministrativeExpenseAdjusted_019e26c2-0007-7ea1-8c2a-15cefd0b7eef" xlink:href="fivn-20260630.xsd#fivn_GeneralAndAdministrativeExpenseAdjusted"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e26c2-0007-74e2-9e5b-845605ba36f8" xlink:to="loc_fivn_GeneralAndAdministrativeExpenseAdjusted_019e26c2-0007-7ea1-8c2a-15cefd0b7eef" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingOtherItemAmount_019e26c2-0007-75f6-a474-03b9b07cd80e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentReportingOtherItemAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e26c2-0007-74e2-9e5b-845605ba36f8" xlink:to="loc_us-gaap_SegmentReportingOtherItemAmount_019e26c2-0007-75f6-a474-03b9b07cd80e" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DepreciationDepletionAndAmortization_019e26c2-0007-7400-bdf4-7d77c9c85509" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DepreciationDepletionAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e26c2-0007-74e2-9e5b-845605ba36f8" xlink:to="loc_us-gaap_DepreciationDepletionAndAmortization_019e26c2-0007-7400-bdf4-7d77c9c85509" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpenseNonoperating_019e26c2-0007-72f2-9f89-190be1fa21a2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestExpenseNonoperating"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e26c2-0007-74e2-9e5b-845605ba36f8" xlink:to="loc_us-gaap_InterestExpenseNonoperating_019e26c2-0007-72f2-9f89-190be1fa21a2" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNonoperatingIncomeExpense_019e26c2-0007-7edd-aa96-92a95c9d39cc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherNonoperatingIncomeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e26c2-0007-74e2-9e5b-845605ba36f8" xlink:to="loc_us-gaap_OtherNonoperatingIncomeExpense_019e26c2-0007-7edd-aa96-92a95c9d39cc" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_019e26c2-0007-7c70-aa83-60c883e41461" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncomeTaxExpenseBenefit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e26c2-0007-74e2-9e5b-845605ba36f8" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_019e26c2-0007-7c70-aa83-60c883e41461" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_019e26c2-0007-7ad8-8432-7e017b9e5d05" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019e26c2-0007-74e2-9e5b-845605ba36f8" xlink:to="loc_us-gaap_NetIncomeLoss_019e26c2-0007-7ad8-8432-7e017b9e5d05" xlink:type="arc" order="10"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.five9.com/role/LeasesNarrativeDetails" xlink:type="simple" xlink:href="fivn-20260630.xsd#LeasesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.five9.com/role/LeasesNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeLeaseDescriptionLineItems_019e26c2-0007-7233-9aa9-5891269387e7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LesseeLeaseDescriptionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeLeaseDescriptionTable_019e26c2-0007-7d6b-9088-886d0e94b05d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LesseeLeaseDescriptionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019e26c2-0007-7233-9aa9-5891269387e7" xlink:to="loc_us-gaap_LesseeLeaseDescriptionTable_019e26c2-0007-7d6b-9088-886d0e94b05d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019e26c2-0007-71d8-b246-17bcf23dd2bc" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LesseeLeaseDescriptionTable_019e26c2-0007-7d6b-9088-886d0e94b05d" xlink:to="loc_srt_RangeAxis_019e26c2-0007-71d8-b246-17bcf23dd2bc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e26c2-0007-71d8-b246-17bcf23dd2bc_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019e26c2-0007-71d8-b246-17bcf23dd2bc" xlink:to="loc_srt_RangeMember_019e26c2-0007-71d8-b246-17bcf23dd2bc_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019e26c2-0007-7723-abea-548b06ca64e8" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019e26c2-0007-71d8-b246-17bcf23dd2bc" xlink:to="loc_srt_RangeMember_019e26c2-0007-7723-abea-548b06ca64e8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019e26c2-0007-7a89-9d8d-93a7062ecdaf" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e26c2-0007-7723-abea-548b06ca64e8" xlink:to="loc_srt_MinimumMember_019e26c2-0007-7a89-9d8d-93a7062ecdaf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019e26c2-0007-7d38-94ac-4fb1097b8c9c" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019e26c2-0007-7723-abea-548b06ca64e8" xlink:to="loc_srt_MaximumMember_019e26c2-0007-7d38-94ac-4fb1097b8c9c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseContractualTermAxis_019e26c2-0007-7d68-b6ae-28c552223a10" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LeaseContractualTermAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LesseeLeaseDescriptionTable_019e26c2-0007-7d6b-9088-886d0e94b05d" xlink:to="loc_us-gaap_LeaseContractualTermAxis_019e26c2-0007-7d68-b6ae-28c552223a10" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseContractualTermDomain_019e26c2-0007-7d68-b6ae-28c552223a10_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LeaseContractualTermDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LeaseContractualTermAxis_019e26c2-0007-7d68-b6ae-28c552223a10" xlink:to="loc_us-gaap_LeaseContractualTermDomain_019e26c2-0007-7d68-b6ae-28c552223a10_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseContractualTermDomain_019e26c2-0007-772b-9882-b4d8309458a1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LeaseContractualTermDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LeaseContractualTermAxis_019e26c2-0007-7d68-b6ae-28c552223a10" xlink:to="loc_us-gaap_LeaseContractualTermDomain_019e26c2-0007-772b-9882-b4d8309458a1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_CommencedAugust2026Member_019f662f-f69d-7015-83e7-76c6bbe038eb" xlink:href="fivn-20260630.xsd#fivn_CommencedAugust2026Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LeaseContractualTermDomain_019e26c2-0007-772b-9882-b4d8309458a1" xlink:to="loc_fivn_CommencedAugust2026Member_019f662f-f69d-7015-83e7-76c6bbe038eb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_CommencedOctober2026Member_019f6630-6d93-7058-b406-595cb83b5bc4" xlink:href="fivn-20260630.xsd#fivn_CommencedOctober2026Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LeaseContractualTermDomain_019e26c2-0007-772b-9882-b4d8309458a1" xlink:to="loc_fivn_CommencedOctober2026Member_019f6630-6d93-7058-b406-595cb83b5bc4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeOperatingLeaseTermOfContract_019e26c2-0007-715c-a202-50b876352dbf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LesseeOperatingLeaseTermOfContract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019e26c2-0007-7233-9aa9-5891269387e7" xlink:to="loc_us-gaap_LesseeOperatingLeaseTermOfContract_019e26c2-0007-715c-a202-50b876352dbf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeOperatingLeaseRenewalTerm_019e26c2-0007-7314-9058-ddbc33371332" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LesseeOperatingLeaseRenewalTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019e26c2-0007-7233-9aa9-5891269387e7" xlink:to="loc_us-gaap_LesseeOperatingLeaseRenewalTerm_019e26c2-0007-7314-9058-ddbc33371332" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_LesseeOperatingLeaseOptionToTerminateNotice_019e26c2-0007-71a3-9000-e51ea6e9c981" xlink:href="fivn-20260630.xsd#fivn_LesseeOperatingLeaseOptionToTerminateNotice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019e26c2-0007-7233-9aa9-5891269387e7" xlink:to="loc_fivn_LesseeOperatingLeaseOptionToTerminateNotice_019e26c2-0007-71a3-9000-e51ea6e9c981" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeFinanceLeaseTermOfContract1_019f6623-d15c-75c4-935e-d6cacb29f04f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LesseeFinanceLeaseTermOfContract1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019e26c2-0007-7233-9aa9-5891269387e7" xlink:to="loc_us-gaap_LesseeFinanceLeaseTermOfContract1_019f6623-d15c-75c4-935e-d6cacb29f04f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RightOfUseAssetObtainedInExchangeForFinanceLeaseLiability_019f6624-4063-7494-a46a-1c3ccb7c2c68" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RightOfUseAssetObtainedInExchangeForFinanceLeaseLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019e26c2-0007-7233-9aa9-5891269387e7" xlink:to="loc_us-gaap_RightOfUseAssetObtainedInExchangeForFinanceLeaseLiability_019f6624-4063-7494-a46a-1c3ccb7c2c68" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_FinanceLeaseLiabilityAcquiredCurrent_019f6624-a426-7e97-a565-c48b123d1cb9" xlink:href="fivn-20260630.xsd#fivn_FinanceLeaseLiabilityAcquiredCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019e26c2-0007-7233-9aa9-5891269387e7" xlink:to="loc_fivn_FinanceLeaseLiabilityAcquiredCurrent_019f6624-a426-7e97-a565-c48b123d1cb9" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_FinanceLeaseLiabilityAcquiredNoncurrent_019f6625-451a-7b52-9c49-240d3919e589" xlink:href="fivn-20260630.xsd#fivn_FinanceLeaseLiabilityAcquiredNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019e26c2-0007-7233-9aa9-5891269387e7" xlink:to="loc_fivn_FinanceLeaseLiabilityAcquiredNoncurrent_019f6625-451a-7b52-9c49-240d3919e589" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseLiability_019e26c2-0007-7012-a96e-a2c877ab73ec" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingLeaseLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019e26c2-0007-7233-9aa9-5891269387e7" xlink:to="loc_us-gaap_OperatingLeaseLiability_019e26c2-0007-7012-a96e-a2c877ab73ec" xlink:type="arc" order="7"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.five9.com/role/LeasesBalanceSheetDisclosuresDetails" xlink:type="simple" xlink:href="fivn-20260630.xsd#LeasesBalanceSheetDisclosuresDetails"/>
  <link:definitionLink xlink:role="http://www.five9.com/role/LeasesBalanceSheetDisclosuresDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeLeaseDescriptionLineItems_019e26c2-0007-7bca-8f25-a6f245531f8d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LesseeLeaseDescriptionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeLeaseDescriptionTable_019e26c2-0007-7359-acec-1fcbab0260b8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LesseeLeaseDescriptionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019e26c2-0007-7bca-8f25-a6f245531f8d" xlink:to="loc_us-gaap_LesseeLeaseDescriptionTable_019e26c2-0007-7359-acec-1fcbab0260b8" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis_019e26c2-0007-716e-83a6-356a61f07d43" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LesseeLeaseDescriptionTable_019e26c2-0007-7359-acec-1fcbab0260b8" xlink:to="loc_srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis_019e26c2-0007-716e-83a6-356a61f07d43" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RealEstateAndAccumulatedDepreciationNameOfPropertyDomain_019e26c2-0007-716e-83a6-356a61f07d43_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RealEstateAndAccumulatedDepreciationNameOfPropertyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis_019e26c2-0007-716e-83a6-356a61f07d43" xlink:to="loc_srt_RealEstateAndAccumulatedDepreciationNameOfPropertyDomain_019e26c2-0007-716e-83a6-356a61f07d43_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RealEstateAndAccumulatedDepreciationNameOfPropertyDomain_019e26c2-0007-7249-a61a-4335e222163f" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RealEstateAndAccumulatedDepreciationNameOfPropertyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis_019e26c2-0007-716e-83a6-356a61f07d43" xlink:to="loc_srt_RealEstateAndAccumulatedDepreciationNameOfPropertyDomain_019e26c2-0007-7249-a61a-4335e222163f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_FixedAssetsExcludingNewFinanceLeaseMember_019e26c2-0007-7015-85cd-adf7fcc208ec" xlink:href="fivn-20260630.xsd#fivn_FixedAssetsExcludingNewFinanceLeaseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RealEstateAndAccumulatedDepreciationNameOfPropertyDomain_019e26c2-0007-7249-a61a-4335e222163f" xlink:to="loc_fivn_FixedAssetsExcludingNewFinanceLeaseMember_019e26c2-0007-7015-85cd-adf7fcc208ec" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_OperatingLeaseAssetsAndLiabilitiesLesseeAbstract_019e26c2-0007-7752-959a-bd3cc9b3fd05" xlink:href="fivn-20260630.xsd#fivn_OperatingLeaseAssetsAndLiabilitiesLesseeAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019e26c2-0007-7bca-8f25-a6f245531f8d" xlink:to="loc_fivn_OperatingLeaseAssetsAndLiabilitiesLesseeAbstract_019e26c2-0007-7752-959a-bd3cc9b3fd05" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseRightOfUseAsset_019e26c2-0007-705d-83f3-1bf1b3180b88" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingLeaseRightOfUseAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fivn_OperatingLeaseAssetsAndLiabilitiesLesseeAbstract_019e26c2-0007-7752-959a-bd3cc9b3fd05" xlink:to="loc_us-gaap_OperatingLeaseRightOfUseAsset_019e26c2-0007-705d-83f3-1bf1b3180b88" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseLiabilityCurrent_019e26c2-0007-7695-ae66-34ddfff3a4db" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingLeaseLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fivn_OperatingLeaseAssetsAndLiabilitiesLesseeAbstract_019e26c2-0007-7752-959a-bd3cc9b3fd05" xlink:to="loc_us-gaap_OperatingLeaseLiabilityCurrent_019e26c2-0007-7695-ae66-34ddfff3a4db" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseLiabilityNoncurrent_019e26c2-0007-7095-9254-e5e55e3f1b43" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingLeaseLiabilityNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fivn_OperatingLeaseAssetsAndLiabilitiesLesseeAbstract_019e26c2-0007-7752-959a-bd3cc9b3fd05" xlink:to="loc_us-gaap_OperatingLeaseLiabilityNoncurrent_019e26c2-0007-7095-9254-e5e55e3f1b43" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseLiability_019e26c2-0007-76a4-8df5-5def1650e07b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingLeaseLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_fivn_OperatingLeaseAssetsAndLiabilitiesLesseeAbstract_019e26c2-0007-7752-959a-bd3cc9b3fd05" xlink:to="loc_us-gaap_OperatingLeaseLiability_019e26c2-0007-76a4-8df5-5def1650e07b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinanceLeaseRightOfUseAssetAfterAccumulatedAmortizationAbstract_019e26c2-0007-7183-a2b3-17da720fde22" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinanceLeaseRightOfUseAssetAfterAccumulatedAmortizationAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019e26c2-0007-7bca-8f25-a6f245531f8d" xlink:to="loc_us-gaap_FinanceLeaseRightOfUseAssetAfterAccumulatedAmortizationAbstract_019e26c2-0007-7183-a2b3-17da720fde22" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinanceLeaseRightOfUseAsset_019e26c2-0007-71e6-8a42-19a73b5c9b03" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinanceLeaseRightOfUseAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinanceLeaseRightOfUseAssetAfterAccumulatedAmortizationAbstract_019e26c2-0007-7183-a2b3-17da720fde22" xlink:to="loc_us-gaap_FinanceLeaseRightOfUseAsset_019e26c2-0007-71e6-8a42-19a73b5c9b03" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinanceLeaseRightOfUseAssetBeforeAccumulatedAmortization_019e26c2-0007-76af-be6f-ae7045b8f1da" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinanceLeaseRightOfUseAssetBeforeAccumulatedAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinanceLeaseRightOfUseAssetAfterAccumulatedAmortizationAbstract_019e26c2-0007-7183-a2b3-17da720fde22" xlink:to="loc_us-gaap_FinanceLeaseRightOfUseAssetBeforeAccumulatedAmortization_019e26c2-0007-76af-be6f-ae7045b8f1da" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinanceLeaseRightOfUseAssetAccumulatedAmortization_019e26c2-0007-7fc2-9e16-91dbfb0f9348" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinanceLeaseRightOfUseAssetAccumulatedAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinanceLeaseRightOfUseAssetAfterAccumulatedAmortizationAbstract_019e26c2-0007-7183-a2b3-17da720fde22" xlink:to="loc_us-gaap_FinanceLeaseRightOfUseAssetAccumulatedAmortization_019e26c2-0007-7fc2-9e16-91dbfb0f9348" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinanceLeaseLiabilityCurrent_019e26c2-0007-749f-9461-3f95b2dc8bf4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinanceLeaseLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinanceLeaseRightOfUseAssetAfterAccumulatedAmortizationAbstract_019e26c2-0007-7183-a2b3-17da720fde22" xlink:to="loc_us-gaap_FinanceLeaseLiabilityCurrent_019e26c2-0007-749f-9461-3f95b2dc8bf4" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinanceLeaseLiabilityNoncurrent_019e26c2-0007-7e06-bfd4-8894a118e367" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinanceLeaseLiabilityNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinanceLeaseRightOfUseAssetAfterAccumulatedAmortizationAbstract_019e26c2-0007-7183-a2b3-17da720fde22" xlink:to="loc_us-gaap_FinanceLeaseLiabilityNoncurrent_019e26c2-0007-7e06-bfd4-8894a118e367" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinanceLeaseLiability_019e26c2-0007-7eee-b955-558855101131" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinanceLeaseLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinanceLeaseRightOfUseAssetAfterAccumulatedAmortizationAbstract_019e26c2-0007-7183-a2b3-17da720fde22" xlink:to="loc_us-gaap_FinanceLeaseLiability_019e26c2-0007-7eee-b955-558855101131" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinanceLeaseRightOfUseAssetStatementOfFinancialPositionExtensibleList_019e26c2-0007-74e4-89ed-56759f28a176" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinanceLeaseRightOfUseAssetStatementOfFinancialPositionExtensibleList"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_019e26c2-0007-7bca-8f25-a6f245531f8d" xlink:to="loc_us-gaap_FinanceLeaseRightOfUseAssetStatementOfFinancialPositionExtensibleList_019e26c2-0007-74e4-89ed-56759f28a176" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.five9.com/role/RestructuringsDetails" xlink:type="simple" xlink:href="fivn-20260630.xsd#RestructuringsDetails"/>
  <link:definitionLink xlink:role="http://www.five9.com/role/RestructuringsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCostAndReserveLineItems_019e26c2-0007-7a8f-a853-1f50f3ee87de" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestructuringCostAndReserveLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_019e26c2-0007-7946-9855-ab9312adff9f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfRestructuringAndRelatedCostsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_019e26c2-0007-7a8f-a853-1f50f3ee87de" xlink:to="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_019e26c2-0007-7946-9855-ab9312adff9f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringPlanAxis_019e26c2-0007-704c-af39-51e82ab32fdd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestructuringPlanAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_019e26c2-0007-7946-9855-ab9312adff9f" xlink:to="loc_us-gaap_RestructuringPlanAxis_019e26c2-0007-704c-af39-51e82ab32fdd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringPlanDomain_019e26c2-0007-704c-af39-51e82ab32fdd_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestructuringPlanDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestructuringPlanAxis_019e26c2-0007-704c-af39-51e82ab32fdd" xlink:to="loc_us-gaap_RestructuringPlanDomain_019e26c2-0007-704c-af39-51e82ab32fdd_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringPlanDomain_019e26c2-0007-7171-a760-623d4df6e7c8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestructuringPlanDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestructuringPlanAxis_019e26c2-0007-704c-af39-51e82ab32fdd" xlink:to="loc_us-gaap_RestructuringPlanDomain_019e26c2-0007-7171-a760-623d4df6e7c8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_A2025PlanMember_019e26c2-0007-716c-a403-c0b80037e352" xlink:href="fivn-20260630.xsd#fivn_A2025PlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringPlanDomain_019e26c2-0007-7171-a760-623d4df6e7c8" xlink:to="loc_fivn_A2025PlanMember_019e26c2-0007-716c-a403-c0b80037e352" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_May2026PlanMember_019f029f-1ffb-70cc-8974-f9584f4fd40e" xlink:href="fivn-20260630.xsd#fivn_May2026PlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringPlanDomain_019e26c2-0007-7171-a760-623d4df6e7c8" xlink:to="loc_fivn_May2026PlanMember_019f029f-1ffb-70cc-8974-f9584f4fd40e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementOfIncomeLocationBalanceAxis_019e26c2-0007-7407-b453-936077375d0d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementOfIncomeLocationBalanceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_019e26c2-0007-7946-9855-ab9312adff9f" xlink:to="loc_us-gaap_StatementOfIncomeLocationBalanceAxis_019e26c2-0007-7407-b453-936077375d0d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCostAndReserveAxis_019e26c2-0007-794f-aff1-14094a67d37b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestructuringCostAndReserveAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_019e26c2-0007-7946-9855-ab9312adff9f" xlink:to="loc_us-gaap_RestructuringCostAndReserveAxis_019e26c2-0007-794f-aff1-14094a67d37b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_019e26c2-0007-794f-aff1-14094a67d37b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TypeOfRestructuringDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_019e26c2-0007-794f-aff1-14094a67d37b" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_019e26c2-0007-794f-aff1-14094a67d37b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_019e26c2-0007-734d-a2f2-441beead3952" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TypeOfRestructuringDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_019e26c2-0007-794f-aff1-14094a67d37b" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_019e26c2-0007-734d-a2f2-441beead3952" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockCompensationPlanMember_019e26c2-0007-7c96-96ac-34da4be1942f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockCompensationPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_019e26c2-0007-734d-a2f2-441beead3952" xlink:to="loc_us-gaap_StockCompensationPlanMember_019e26c2-0007-7c96-96ac-34da4be1942f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringAndRelatedCostNumberOfPositionsEliminatedPeriodPercent_019e26c2-0007-7941-a116-14284d83b442" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestructuringAndRelatedCostNumberOfPositionsEliminatedPeriodPercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_019e26c2-0007-7a8f-a853-1f50f3ee87de" xlink:to="loc_us-gaap_RestructuringAndRelatedCostNumberOfPositionsEliminatedPeriodPercent_019e26c2-0007-7941-a116-14284d83b442" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCharges_019e26c2-0007-70ae-a685-a8c4c2a0033d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestructuringCharges"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_019e26c2-0007-7a8f-a853-1f50f3ee87de" xlink:to="loc_us-gaap_RestructuringCharges_019e26c2-0007-70ae-a685-a8c4c2a0033d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsForRestructuring_019e26c2-0007-7e76-99ca-3515612fd282" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PaymentsForRestructuring"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_019e26c2-0007-7a8f-a853-1f50f3ee87de" xlink:to="loc_us-gaap_PaymentsForRestructuring_019e26c2-0007-7e76-99ca-3515612fd282" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAcceleratedCompensationCost_019e26c2-0007-736e-a0b6-decc2b77e609" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAcceleratedCompensationCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_019e26c2-0007-7a8f-a853-1f50f3ee87de" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAcceleratedCompensationCost_019e26c2-0007-736e-a0b6-decc2b77e609" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringIncurredCostStatementOfIncomeOrComprehensiveIncomeExtensibleEnumeration_019f367d-6b22-7671-aaaf-557beb74ec20" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestructuringIncurredCostStatementOfIncomeOrComprehensiveIncomeExtensibleEnumeration"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_019e26c2-0007-7a8f-a853-1f50f3ee87de" xlink:to="loc_us-gaap_RestructuringIncurredCostStatementOfIncomeOrComprehensiveIncomeExtensibleEnumeration_019f367d-6b22-7671-aaaf-557beb74ec20" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.five9.com/role/RestructuringsDetails_1" xlink:type="simple" xlink:href="fivn-20260630.xsd#RestructuringsDetails_1"/>
  <link:definitionLink xlink:role="http://www.five9.com/role/RestructuringsDetails_1" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCostAndReserveLineItems_019e26c2-0007-7a8f-a853-1f50f3ee87de" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestructuringCostAndReserveLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_019e26c2-0007-7946-9855-ab9312adff9f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfRestructuringAndRelatedCostsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_019e26c2-0007-7a8f-a853-1f50f3ee87de" xlink:to="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_019e26c2-0007-7946-9855-ab9312adff9f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringPlanAxis_019e26c2-0007-704c-af39-51e82ab32fdd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestructuringPlanAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_019e26c2-0007-7946-9855-ab9312adff9f" xlink:to="loc_us-gaap_RestructuringPlanAxis_019e26c2-0007-704c-af39-51e82ab32fdd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringPlanDomain_019e26c2-0007-704c-af39-51e82ab32fdd_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestructuringPlanDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestructuringPlanAxis_019e26c2-0007-704c-af39-51e82ab32fdd" xlink:to="loc_us-gaap_RestructuringPlanDomain_019e26c2-0007-704c-af39-51e82ab32fdd_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringPlanDomain_019e26c2-0007-7171-a760-623d4df6e7c8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestructuringPlanDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestructuringPlanAxis_019e26c2-0007-704c-af39-51e82ab32fdd" xlink:to="loc_us-gaap_RestructuringPlanDomain_019e26c2-0007-7171-a760-623d4df6e7c8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_A2025PlanMember_019e26c2-0007-716c-a403-c0b80037e352" xlink:href="fivn-20260630.xsd#fivn_A2025PlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringPlanDomain_019e26c2-0007-7171-a760-623d4df6e7c8" xlink:to="loc_fivn_A2025PlanMember_019e26c2-0007-716c-a403-c0b80037e352" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_May2026PlanMember_019f029f-1ffb-70cc-8974-f9584f4fd40e" xlink:href="fivn-20260630.xsd#fivn_May2026PlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringPlanDomain_019e26c2-0007-7171-a760-623d4df6e7c8" xlink:to="loc_fivn_May2026PlanMember_019f029f-1ffb-70cc-8974-f9584f4fd40e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCostAndReserveAxis_019e26c2-0007-794f-aff1-14094a67d37b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestructuringCostAndReserveAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_019e26c2-0007-7946-9855-ab9312adff9f" xlink:to="loc_us-gaap_RestructuringCostAndReserveAxis_019e26c2-0007-794f-aff1-14094a67d37b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_019e26c2-0007-794f-aff1-14094a67d37b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TypeOfRestructuringDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_019e26c2-0007-794f-aff1-14094a67d37b" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_019e26c2-0007-794f-aff1-14094a67d37b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_019e26c2-0007-734d-a2f2-441beead3952" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TypeOfRestructuringDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_019e26c2-0007-794f-aff1-14094a67d37b" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_019e26c2-0007-734d-a2f2-441beead3952" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockCompensationPlanMember_019e26c2-0007-7c96-96ac-34da4be1942f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockCompensationPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_019e26c2-0007-734d-a2f2-441beead3952" xlink:to="loc_us-gaap_StockCompensationPlanMember_019e26c2-0007-7c96-96ac-34da4be1942f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringAndRelatedCostNumberOfPositionsEliminatedPeriodPercent_019e26c2-0007-7941-a116-14284d83b442" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestructuringAndRelatedCostNumberOfPositionsEliminatedPeriodPercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_019e26c2-0007-7a8f-a853-1f50f3ee87de" xlink:to="loc_us-gaap_RestructuringAndRelatedCostNumberOfPositionsEliminatedPeriodPercent_019e26c2-0007-7941-a116-14284d83b442" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCharges_019e26c2-0007-70ae-a685-a8c4c2a0033d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestructuringCharges"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_019e26c2-0007-7a8f-a853-1f50f3ee87de" xlink:to="loc_us-gaap_RestructuringCharges_019e26c2-0007-70ae-a685-a8c4c2a0033d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsForRestructuring_019e26c2-0007-7e76-99ca-3515612fd282" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PaymentsForRestructuring"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_019e26c2-0007-7a8f-a853-1f50f3ee87de" xlink:to="loc_us-gaap_PaymentsForRestructuring_019e26c2-0007-7e76-99ca-3515612fd282" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAcceleratedCompensationCost_019e26c2-0007-736e-a0b6-decc2b77e609" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAcceleratedCompensationCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_019e26c2-0007-7a8f-a853-1f50f3ee87de" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAcceleratedCompensationCost_019e26c2-0007-736e-a0b6-decc2b77e609" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringIncurredCostStatementOfIncomeOrComprehensiveIncomeExtensibleEnumeration_019f367d-6b22-7671-aaaf-557beb74ec20" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestructuringIncurredCostStatementOfIncomeOrComprehensiveIncomeExtensibleEnumeration"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_019e26c2-0007-7a8f-a853-1f50f3ee87de" xlink:to="loc_us-gaap_RestructuringIncurredCostStatementOfIncomeOrComprehensiveIncomeExtensibleEnumeration_019f367d-6b22-7671-aaaf-557beb74ec20" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fasb.org/srt/role/srt-eedm/ExtensibleEnumerationLists" xlink:type="simple" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-roles-2026.xsd#eedm1"/>
  <link:definitionLink xlink:role="http://fasb.org/srt/role/srt-eedm/ExtensibleEnumerationLists" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsAbstract" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssetsAbstract"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet" xlink:type="arc" order="101"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://xbrl.sec.gov/ecd/role/IndividualsOnly" xlink:type="simple" xlink:href="https://xbrl.sec.gov/ecd/2026/ecd-2026.xsd#IndividualsOnly"/>
  <link:definitionLink xlink:role="http://xbrl.sec.gov/ecd/role/IndividualsOnly" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_ecd_AllIndividualsMember" xlink:href="https://xbrl.sec.gov/ecd/2026/ecd-2026.xsd#ecd_AllIndividualsMember"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_PanosKozanianMember_019e26c1-fd87-7684-b6e7-436d374376fa" xlink:href="fivn-20260630.xsd#fivn_PanosKozanianMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecd_AllIndividualsMember" xlink:to="loc_fivn_PanosKozanianMember_019e26c1-fd87-7684-b6e7-436d374376fa" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_LeenaMansharamaniMember_019f3646-eb7f-7129-bb2f-bdac6973df1b" xlink:href="fivn-20260630.xsd#fivn_LeenaMansharamaniMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecd_AllIndividualsMember" xlink:to="loc_fivn_LeenaMansharamaniMember_019f3646-eb7f-7129-bb2f-bdac6973df1b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_fivn_TiffanyMeriweatherMember_019f3648-ef1c-730d-ac7e-c2770880e9a0" xlink:href="fivn-20260630.xsd#fivn_TiffanyMeriweatherMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecd_AllIndividualsMember" xlink:to="loc_fivn_TiffanyMeriweatherMember_019f3648-ef1c-730d-ac7e-c2770880e9a0" xlink:type="arc" order="4"/>
  </link:definitionLink>
</link:linkbase>
