SCHEDULE 13G/A 0000830588 XXXXXXXX LIVE 5 Series A Common Stock, par value $0.001 per share 09/30/2025 0001835830 Klaviyo, Inc. 49845K101 125 Summer Street, Floor 6 Boston MA 02110 Rule 13d-1(d) Summit Partners Growth Equity Fund IX-A, L.P. b DE 0.00 11142414 0.00 11142414 11142414 N 7.11 PN Represents 11,142,414 shares of Series A Common Stock ("Series A Shares") issuable in respect of 11,142,414 shares of Series B Common Stock ("Series B Shares"). Calculated based upon 137,856,147 Series A Shares outstanding as of October 31, 2025 as reported on the Issuer's Quarterly Report on Form 10-Q (the "10-Q") filed with the Securities and Exchange Commission ("SEC") on November 5, 2025, as increased by an aggregate of 18,852,778 Series A Shares issuable in respect of the 18,852,778 Series B Shares held by the Summit Holders (as defined in Item 2). Summit Partners Growth Equity Fund IX-B, L.P. b DE 0.00 6957168 0.00 6957168 6957168 N 4.44 PN Represents 6,957,168 Series A Shares issuable in respect of 6,957,168 Series B Shares. Calculated based upon 137,856,147 Series A Shares outstanding as of October 31, 2025 as reported on the Issuer's 10-Q filed with the SEC on November 5, 2025, as increased by an aggregate of 18,852,778 Series A Shares issuable in respect of the 18,852,778 Series B Shares held by the Summit Holders (as defined in Item 2). Summit Partners Co-Invest (Kiwi), L.P. b DE 0.00 687035 0.00 687035 687035 N 0.44 PN Represents 687,035 Series A Shares issuable in respect of 687,035 Series B Shares. Calculated based upon 137,856,147 Series A Shares outstanding as of October 31, 2025 as reported on the Issuer's 10-Q filed with the SEC on November 5, 2025, as increased by an aggregate of 18,852,778 Series A Shares issuable in respect of the 18,852,778 Series B Shares held by the Summit Holders (as defined in Item 2). Summit Investors GE IX/VC IV, LLC b DE 0.00 58561 0.00 58561 58561 N 0.04 OO Represents 58,561 Series A Shares issuable in respect of 58,561 Series B Shares. Calculated based upon 137,856,147 Series A Shares outstanding as of October 31, 2025 as reported on the Issuer's 10-Q filed with the SEC on November 5, 2025, as increased by an aggregate of 18,852,778 Series A Shares issuable in respect of the 18,852,778 Series B Shares held by the Summit Holders (as defined in Item 2). Summit Investors GE IX/VC IV (UK), L.P. b E9 0.00 7600 0.00 7600 7600 N 0.01 PN Represents 7,600 Series A Shares issuable in respect of 7,600 Series B Shares. Calculated based upon 137,856,147 Series A Shares outstanding as of October 31, 2025 as reported on the Issuer's 10-Q filed with the SEC on November 5, 2025, as increased by an aggregate of 18,852,778 Series A Shares issuable in respect of the 18,852,778 Series B Shares held by the Summit Holders (as defined in Item 2). Summit Partners, L.P. b DE 0.00 18852778 0.00 18852778 18852778 N 12.03 PN Represents 18,852,778 Series A Shares issuable in respect of 18,852,778 Series B Shares. Calculated based upon 137,856,147 Series A Shares outstanding as of October 31, 2025 as reported on the Issuer's 10-Q filed with the SEC on November 5, 2025, as increased by an aggregate of 18,852,778 Series A Shares issuable in respect of the 18,852,778 Series B Shares held by the Summit Holders (as defined in Item 2). Klaviyo, Inc. 125 Summer Street, Floor 6, Boston, MA 02110 This statement is filed by the entities and persons listed below, all of whom together are referred to herein as the "Reporting Persons": (i) Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") (ii) Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B") (iii) Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund") (iv) Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") (v) Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)," and together with each of the foregoing, the "Summit Holders") (vi) Summit Partners, L.P. 222 Berkeley Street, 18th Floor Boston, MA 02116 See responses to Item 4 on each cover page. Y See responses to Item 9 on each cover page. The aggregate Series A Shares beneficially owned by the Reporting Persons and reflected in this statement represents 12.03% of the outstanding Series A Shares. Such calculation is based upon 137,856,147 Series A Shares outstanding as of October 31, 2025 as reported on the Issuer's 10-Q filed with the SEC on November 5, 2025, as increased by an aggregate of 18,852,778 Series A Shares issuable in respect of the 18,852,778 Series B Shares held by the Summit Holders. In this regard, the reported securities are directly held by Fund IX-A (11,142,414 Series A Shares issuable in respect of 11,142,414 Series B Shares), Fund IX-B (6,957,168 Series A Shares issuable in respect of 6,957,168 Series B Shares), Kiwi Co-Invest Fund (687,035 Series A Shares issuable in respect of 687,035 Series B Shares), Fund IX/VC IV (58,561 Series A Shares issuable in respect of 58,561 Series B Shares), and Fund IX/VC IV (UK) (7,600 Series A Shares issuable in respect of 7,600 Series B Shares). Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, L.P., which is the general partner of Fund IX-A and Fund IX-B. Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Kiwi Co-Invest Fund. Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Fund IX/VC IV and the general partner of Fund IX/VC IV (UK). Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the Series B Shares held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P. The investment committee of Summit Partners, L.P. is currently composed of Peter Chung, Scott Collins and Peter Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the reported securities (except to the extent such securities are directly held by such entity). The filing of this statement shall not be construed as an admission that the Reporting Persons or any of the foregoing are the beneficial owners of any of the securities covered by this statement. See responses to Item 11 on each cover page See responses to Item 5 on each cover page. See responses to Item 6 on each cover page. See responses to Item 7 on each cover page. See responses to Item 8 on each cover page. Y Y Y Y Y Y Summit Partners Growth Equity Fund IX-A, L.P. /s/ Adam H. Hennessey, as POA 11/13/2025 Summit Partners Growth Equity Fund IX-B, L.P. /s/ Adam H. Hennessey, as POA 11/13/2025 Summit Partners Co-Invest (Kiwi), L.P. /s/ Adam H. Hennessey, as POA 11/13/2025 Summit Investors GE IX/VC IV, LLC /s/ Adam H. Hennessey, as POA 11/13/2025 Summit Investors GE IX/VC IV (UK), L.P. /s/ Adam H. Hennessey, as POA 11/13/2025 Summit Partners, L.P. /s/ Adam H. Hennessey, as POA 11/13/2025 Exhibit A Joint Filing Agreement, dated as of January 31, 2024, incorporated by reference to the Schedule 13G filed by the Reporting Persons on January 31, 2024 Exhibit B Powers of Attorney, incorporated herein by reference to of the statement on Schedule 13G filed by the Reporting Persons on January 31, 2024