SCHEDULE 13G/A 0000830588 XXXXXXXX LIVE 4 Series A Common Stock, par value $0.001 per share 06/30/2025 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 18219907.00 0.00 18219907.00 18219907.00 N 12.33 PN Represents 18,219,907 shares of Series A Common Stock ("Series A Shares") issuable in respect of 18,219,907 shares of Series B Common Stock ("Series B Shares"). Calculated based upon 116,897,730 Series A Shares outstanding as of July 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 August 5, 2025, as increased by an aggregate of 30,827,778 Series A Shares issuable in respect of the 30,827,778 Series B Shares held by the Summit Holders (as defined in Item 2). Summit Partners Growth Equity Fund IX-B, L.P. b DE 0.00 11376255.00 0.00 11376255.00 11376255.00 N 7.70 PN Represents 11,376,255 Series A Shares issuable in respect of 11,376,255 Series B Shares. Calculated based upon 116,897,730 Series A Shares outstanding as of July 31, 2025 as reported on the Issuer's 10-Q filed with the SEC on August 5, 2025, as increased by an aggregate of 30,827,778 Series A Shares issuable in respect of the 30,827,778 Series B Shares held by the Summit Holders (as defined in Item 2). Summit Partners Co-Invest (Kiwi), L.P. b DE 0.00 1123429.00 0.00 1123429.00 1123429.00 N 0.76 PN Represents 1,123,429 Series A Shares issuable in respect of 1,123,429 Series B Shares. Calculated based upon 116,897,730 Series A Shares outstanding as of July 31, 2025 as reported on the Issuer's 10-Q filed with the SEC on August 5, 2025, as increased by an aggregate of 30,827,778 Series A Shares issuable in respect of the 30,827,778 Series B Shares held by the Summit Holders (as defined in Item 2). Summit Investors GE IX/VC IV, LLC b DE 0.00 95759.00 0.00 95759.00 95759.00 N 0.06 OO Represents 95,759 Series A Shares issuable in respect of 95,759 Series B Shares. Calculated based upon 116,897,730 Series A Shares outstanding as of July 31, 2025 as reported on the Issuer's 10-Q filed with the SEC on August 5, 2025, as increased by an aggregate of 30,827,778 Series A Shares issuable in respect of the 30,827,778 Series B Shares held by the Summit Holders (as defined in Item 2). Summit Investors GE IX/VC IV (UK), L.P. b E9 0.00 12428.00 0.00 12428.00 12428.00 N 0.01 PN Represents 12,428 Series A Shares issuable in respect of 12,428 Series B Shares. Calculated based upon 116,897,730 Series A Shares outstanding as of July 31, 2025 as reported on the Issuer's 10-Q filed with the SEC on August 5, 2025, as increased by an aggregate of 30,827,778 Series A Shares issuable in respect of the 30,827,778 Series B Shares held by the Summit Holders (as defined in Item 2). Summit Partners, L.P. b DE 0.00 30827778.00 0.00 30827778.00 30827778.00 N 20.87 PN Represents 30,827,778 Series A Shares issuable in respect of 30,827,778 Series B Shares. Calculated based upon 116,897,730 Series A Shares outstanding as of July 31, 2025 as reported on the Issuer's 10-Q filed with the SEC on August 5, 2025, as increased by an aggregate of 30,827,778 Series A Shares issuable in respect of the 30,827,778 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 aggregate Series A Shares beneficially owned by the Reporting Persons and reflected in this statement represents 20.87% of the outstanding Series A Shares. Such calculation is based upon 116,897,730 Series A Shares outstanding as of July 31, 2025 as reported on the Issuer's 10-Q filed with the SEC on August 5, 2025, as increased by an aggregate of 30,827,778 Series A Shares issuable in respect of the 30,827,778 Series B Shares held by the Summit Holders. In this regard, the reported securities are directly held by Fund IX-A (18,219,907 Series A Shares issuable in respect of 18,219,907 Series B Shares), Fund IX-B (11,376,255 Series A Shares issuable in respect of 11,376,255 Series B Shares), Kiwi Co-Invest Fund (1,123,429 Series A Shares issuable in respect of 1,123,429 Series B Shares), Fund IX/VC IV (95,759 Series A Shares issuable in respect of 95,759 Series B Shares), and Fund IX/VC IV (UK) (12,428 Series A Shares issuable in respect of 12,428 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 08/08/2025 Summit Partners Growth Equity Fund IX-B, L.P. /s/ Adam H. Hennessey, as POA 08/08/2025 Summit Partners Co-Invest (Kiwi), L.P. /s/ Adam H. Hennessey, as POA 08/08/2025 Summit Investors GE IX/VC IV, LLC /s/ Adam H. Hennessey, as POA 08/08/2025 Summit Investors GE IX/VC IV (UK), L.P. /s/ Adam H. Hennessey, as POA 08/08/2025 Summit Partners, L.P. /s/ Adam H. Hennessey, as POA 08/08/2025 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