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<headerData>
<submissionType>144</submissionType>
<filerInfo>
<filer>
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<cik>0001830877</cik>
<ccc>XXXXXXXX</ccc>
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<liveTestFlag>LIVE</liveTestFlag>
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<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 Partners Co-Invest (Kiwi), LP </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>
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<noOfUnitsSold>127547</noOfUnitsSold>
<aggregateMarketValue>4712861.65</aggregateMarketValue>
<noOfUnitsOutstanding>78803963</noOfUnitsOutstanding>
<approxSaleDate>11/14/2024</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>1983585</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 Partners Co-Invest (Kiwi), 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>08/16/2024</saleDate>
<amountOfSecuritiesSold>8841</amountOfSecuritiesSold>
<grossProceeds>280960.61</grossProceeds>
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<securitiesSoldInPast3Months>
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<name>Summit Partners Co-Invest (Kiwi), L.P.</name>
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<saleDate>08/20/2024</saleDate>
<amountOfSecuritiesSold>4953</amountOfSecuritiesSold>
<grossProceeds>158226.1</grossProceeds>
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<name>Summit Partners Co-Invest (Kiwi), L.P.</name>
<address>
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<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>
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<securitiesClassTitle>Series A common stock, par value $0.001 per share</securitiesClassTitle>
<saleDate>08/21/2024</saleDate>
<amountOfSecuritiesSold>4149</amountOfSecuritiesSold>
<grossProceeds>131773.51</grossProceeds>
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<securitiesSoldInPast3Months>
<sellerDetails>
<name>Summit Partners Co-Invest (Kiwi), L.P.</name>
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<street1 xmlns="http://www.sec.gov/edgar/common">222 Berkeley St 18th FL</street1>
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<zipCode xmlns="http://www.sec.gov/edgar/common">02116</zipCode>
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<securitiesClassTitle>Series A common stock, par value $0.001 per share</securitiesClassTitle>
<saleDate>08/22/2024</saleDate>
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<grossProceeds>95510.08</grossProceeds>
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<name>Summit Partners Co-Invest (Kiwi), L.P.</name>
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<zipCode xmlns="http://www.sec.gov/edgar/common">02116</zipCode>
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<securitiesClassTitle>Series A common stock, par value $0.001 per share</securitiesClassTitle>
<saleDate>08/23/2024</saleDate>
<amountOfSecuritiesSold>11138</amountOfSecuritiesSold>
<grossProceeds>360132.86</grossProceeds>
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<securitiesSoldInPast3Months>
<sellerDetails>
<name>Summit Partners Co-Invest (Kiwi), L.P.</name>
<address>
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<zipCode xmlns="http://www.sec.gov/edgar/common">02116</zipCode>
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<securitiesClassTitle>Series A common stock, par value $0.001 per share</securitiesClassTitle>
<saleDate>08/26/2024</saleDate>
<amountOfSecuritiesSold>3298</amountOfSecuritiesSold>
<grossProceeds>106422.97</grossProceeds>
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<sellerDetails>
<name>Summit Partners Co-Invest (Kiwi), L.P.</name>
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<securitiesClassTitle>Series A common stock, par value $0.001 per share</securitiesClassTitle>
<saleDate>08/27/2024</saleDate>
<amountOfSecuritiesSold>2458</amountOfSecuritiesSold>
<grossProceeds>79378.73</grossProceeds>
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<securitiesSoldInPast3Months>
<sellerDetails>
<name>Summit Partners Co-Invest (Kiwi), L.P.</name>
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<securitiesClassTitle>Series A common stock, par value $0.001 per share</securitiesClassTitle>
<saleDate>08/28/2024</saleDate>
<amountOfSecuritiesSold>1000</amountOfSecuritiesSold>
<grossProceeds>31676.81</grossProceeds>
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<name>Summit Partners Co-Invest (Kiwi), L.P.</name>
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<securitiesClassTitle>Series A common stock, par value $0.001 per share</securitiesClassTitle>
<saleDate>08/29/2024</saleDate>
<amountOfSecuritiesSold>617</amountOfSecuritiesSold>
<grossProceeds>19463.95</grossProceeds>
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<sellerDetails>
<name>Summit Partners Co-Invest (Kiwi), L.P.</name>
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<securitiesClassTitle>Series A common stock, par value $0.001 per share</securitiesClassTitle>
<saleDate>08/30/2024</saleDate>
<amountOfSecuritiesSold>47</amountOfSecuritiesSold>
<grossProceeds>1487.08</grossProceeds>
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<name>Summit Partners Co-Invest (Kiwi), L.P.</name>
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<saleDate>09/11/2024</saleDate>
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<name>Summit Partners Co-Invest (Kiwi), L.P.</name>
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<saleDate>09/12/2024</saleDate>
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<securitiesSoldInPast3Months>
<sellerDetails>
<name>Summit Partners Co-Invest (Kiwi), L.P.</name>
<address>
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<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>
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<securitiesClassTitle>Series A common stock, par value $0.001 per share</securitiesClassTitle>
<saleDate>09/13/2024</saleDate>
<amountOfSecuritiesSold>4138</amountOfSecuritiesSold>
<grossProceeds>130229.24</grossProceeds>
</securitiesSoldInPast3Months>
<noticeSignature>
<noticeDate>11/14/2024</noticeDate>
<signature>/s/ Adam Hennessey</signature>
</noticeSignature>
</formData>
</edgarSubmission>
