content: <?xml version="1.0" encoding="UTF-8"?><edgarSubmission xmlns="http://www.sec.gov/edgar/ownership" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
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  <submissionType>144</submissionType>
  <filerInfo>
  <filer>
  <filerCredentials>
  <cik>0001846709</cik>
  <ccc>XXXXXXXX</ccc>
  </filerCredentials>
  </filer>
  <liveTestFlag>LIVE</liveTestFlag>
  </filerInfo>
  </headerData>
  <formData>
  <issuerInfo>
  <issuerCik>0001835830</issuerCik>
  <issuerName>Klaviyo, Inc.</issuerName>
  <secFileNumber>001-41806</secFileNumber>
  <issuerAddress>
  <street1 xmlns="http://www.sec.gov/edgar/common">125 Summer Street, 6th Floor</street1>
  <city xmlns="http://www.sec.gov/edgar/common">Boston</city>
  <stateOrCountry xmlns="http://www.sec.gov/edgar/common">MA</stateOrCountry>
  <zipCode xmlns="http://www.sec.gov/edgar/common">02110</zipCode>
  </issuerAddress>
  <issuerContactPhone>617-213-1788</issuerContactPhone>
  <nameOfPersonForWhoseAccountTheSecuritiesAreToBeSold>SUMMIT INVESTORS GE IX/VC IV (UK), L.P. </nameOfPersonForWhoseAccountTheSecuritiesAreToBeSold>
  <relationshipsToIssuer>
  <relationshipToIssuer>Owner</relationshipToIssuer>
  </relationshipsToIssuer>
  </issuerInfo>
  <securitiesInformation>
  <securitiesClassTitle>Series A common stock, par value $0.001 per share</securitiesClassTitle>
  <brokerOrMarketmakerDetails>
  <name>Barclays Capital Inc.</name>
  <address>
  <street1 xmlns="http://www.sec.gov/edgar/common">745 7th Avenue</street1>
  <city xmlns="http://www.sec.gov/edgar/common">New York</city>
  <stateOrCountry xmlns="http://www.sec.gov/edgar/common">NY</stateOrCountry>
  <zipCode xmlns="http://www.sec.gov/edgar/common">10019</zipCode>
  </address>
  </brokerOrMarketmakerDetails>
  <noOfUnitsSold>1613</noOfUnitsSold>
  <aggregateMarketValue>55648.5</aggregateMarketValue>
  <noOfUnitsOutstanding>94683729</noOfUnitsOutstanding>
  <approxSaleDate>05/19/2025</approxSaleDate>
  <securitiesExchangeName>NYSE</securitiesExchangeName>
  </securitiesInformation>
  <securitiesToBeSold>
  <securitiesClassTitle>Series A common stock, par value $0.001 per share</securitiesClassTitle>
  <acquiredDate>09/20/2023</acquiredDate>
  <natureOfAcquisitionTransaction>Pre-IPO transactions.</natureOfAcquisitionTransaction>
  <nameOfPersonfromWhomAcquired>Klaviyo, Inc.</nameOfPersonfromWhomAcquired>
  <isGiftTransaction>N</isGiftTransaction>
  <amountOfSecuritiesAcquired>21945</amountOfSecuritiesAcquired>
  <paymentDate>09/20/2023</paymentDate>
  <natureOfPayment>Equity interests in the Issuer were purchased pre-IPO.</natureOfPayment>
  </securitiesToBeSold>
  <nothingToReportFlagOnSecuritiesSoldInPast3Months>N</nothingToReportFlagOnSecuritiesSoldInPast3Months>
  <securitiesSoldInPast3Months>
  <sellerDetails>
  <name>SUMMIT INVESTORS GE IX/VC IV (UK), L.P.</name>
  <address>
  <street1 xmlns="http://www.sec.gov/edgar/common">222 Berkeley St 18th FL</street1>
  <city xmlns="http://www.sec.gov/edgar/common">Boston</city>
  <stateOrCountry xmlns="http://www.sec.gov/edgar/common">MA</stateOrCountry>
  <zipCode xmlns="http://www.sec.gov/edgar/common">02116</zipCode>
  </address>
  </sellerDetails>
  <securitiesClassTitle>Series A common stock, par value $0.001 per share</securitiesClassTitle>
  <saleDate>03/14/2025</saleDate>
  <amountOfSecuritiesSold>806</amountOfSecuritiesSold>
  <grossProceeds>25792</grossProceeds>
  </securitiesSoldInPast3Months>
  <noticeSignature>
  <noticeDate>05/19/2025</noticeDate>
  <signature>/s/ Adam Hennessey</signature>
  </noticeSignature>
  </formData>
  </edgarSubmission>
metadata_documents: [{"type": "144", "sequence": "1", "filename": "primary_doc.xml", "secsgml_size_bytes": 3461}]
text: 144
  0001846709
  XXXXXXXX
  LIVE
  0001835830
  Klaviyo, Inc.
  001-41806
  125 Summer Street, 6th Floor
  Boston
  MA
  02110
  617-213-1788
  SUMMIT INVESTORS GE IX/VC IV (UK), L.P.
  Owner
  Series A common stock, par value $0.001 per share
  Barclays Capital Inc.
  745 7th Avenue
  New York
  NY
  10019
  1613
  55648.5
  94683729
  05/19/2025
  NYSE
  Series A common stock, par value $0.001 per share
  09/20/2023
  Pre-IPO transactions.
  Klaviyo, Inc.
  N
  21945
  09/20/2023
  Equity interests in the Issuer were purchased pre-IPO.
  N
  SUMMIT INVESTORS GE IX/VC IV (UK), L.P.
  222 Berkeley St 18th FL
  Boston
  MA
  02116
  Series A common stock, par value $0.001 per share
  03/14/2025
  806
  25792
  05/19/2025
  /s/ Adam Hennessey