SCHEDULE 13G/A 0001193125-24-254194 0000830588 XXXXXXXX LIVE 2 Series A Common Stock, par value $0.001 per share 12/31/2024 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 24130130 0 24130130 24130130 N 20.17 PN Represents 24,130,130 shares of Series A Common Stock ("Series A Shares") issuable in respect of 24,130,130 shares of Series B Common Stock ("Series B Shares"). Calculated based upon 78,803,963 Series A Shares outstanding as of October 31, 2024, as reported on the Issuer's Form 10-Q filed with the Securities and Exchange Commission on September 30, 2024, as increased by an aggregate of 40,827,778 Series A Shares issuable in respect of the 40,827,778 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 15066515 0 15066515 15066515 N 12.59 PN Represents 15,066,515 Series A Shares issuable in respect of 15,066,515 Series B Shares. Calculated based upon 78,803,963 Series A Shares outstanding as of October 31, 2024, as reported on the Issuer's Form 10-Q filed with the Securities and Exchange Commission on September 30, 2024, as increased by an aggregate of 40,827,778 Series A Shares issuable in respect of the 40,827,778 Series B Shares held by the Summit Holders (as defined in Item 2). Summit Partners Co-Invest (Kiwi), L.P. b DE 0 1487851 0 1487851 1487851 N 1.24 PN Represents 1,487,851 Series A Shares issuable in respect of 1,487,851 Series B Shares. Calculated based upon 78,803,963 Series A Shares outstanding as of October 31, 2024, as reported on the Issuer's Form 10-Q filed with the Securities and Exchange Commission on September 30, 2024, as increased by an aggregate of 40,827,778 Series A Shares issuable in respect of the 40,827,778 Series B Shares held by the Summit Holders (as defined in Item 2). Summit Investors GE IX/VC IV, LLC b DE 0 126822 0 126822 126822 N 0.11 OO Represents 126,822 Series A Shares issuable in respect of 126,822 Series B Shares. Calculated based upon 78,803,963 Series A Shares outstanding as of October 31, 2024, as reported on the Issuer's Form 10-Q filed with the Securities and Exchange Commission on September 30, 2024, as increased by an aggregate of 40,827,778 Series A Shares issuable in respect of the 40,827,778 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 16460 0 16460 16460 N 0.01 PN Represents 16,460 Series A Shares issuable in respect of 16,460 Series B Shares. Calculated based upon 78,803,963 Series A Shares outstanding as of October 31, 2024, as reported on the Issuer's Form 10-Q filed with the Securities and Exchange Commission on September 30, 2024, as increased by an aggregate of 40,827,778 Series A Shares issuable in respect of the 40,827,778 Series B Shares held by the Summit Holders (as defined in Item 2). Summit Partners, L.P. b DE 0 40827778 0 40827778 40827778 N 34.13 PN Represents 40,827,778 Series A Shares issuable in respect of 40,827,778 Series B Shares. Calculated based upon 78,803,963 Series A Shares outstanding as of October 31, 2024, as reported on the Issuer's Form 10-Q filed with the Securities and Exchange Commission on September 30, 2024, as increased by an aggregate of 40,827,778 Series A Shares issuable in respect of the 40,827,778 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 reported securities are held by Fund IX-A (24,130,130 Series A Shares issuable in respect of 24,130,130 Series B Shares), Fund IX-B (15,066,515 Series A Shares issuable in respect of 15,066,515 Series B Shares), Kiwi Co-Invest Fund (1,487,851 Series A Shares issuable in respect of 1,487,851 Series B Shares), Fund IX/VC IV (126,822 Series A Shares issuable in respect of 126,822 Series B Shares), and Fund IX/VC IV (UK) (16,460 Series A Shares issuable in respect of 16,460 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, LP, 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 02/07/2025 Summit Partners Growth Equity Fund IX-B, L.P. /s/ Adam H. Hennessey, as POA 02/07/2025 Summit Partners Co-Invest (Kiwi), L.P. /s/ Adam H. Hennessey, as POA 02/07/2025 Summit Investors GE IX/VC IV, LLC /s/ Adam H. Hennessey, as POA 02/07/2025 Summit Investors GE IX/VC IV (UK), L.P. /s/ Adam H. Hennessey, as POA 02/07/2025 Summit Partners, L.P. /s/ Adam H. Hennessey, as POA 02/07/2025 EXHIBIT LIST 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