SCHEDULE 13G/A 0001991125 XXXXXXXX LIVE 1 Series A common stock, par value $0.001 per share (the "Series A Common Stock") 09/30/2025 0001835830 Klaviyo, Inc. (the "Issuer") 49845K101 125 Summer Street, 6th Floor Boston MA 02110 Rule 13d-1(d) Hallen Ed X1 32879098.00 0.00 32879098.00 0.00 32879098.00 N 19.4 IN Klaviyo, Inc. (the "Issuer") 125 Summer Street, 6th Floor, Boston, MA, 02110 This statement is filed on behalf of Ed Hallen (the "Reporting Person"). The business address of the Reporting Person is c/o Klaviyo, Inc., 125 Summer Street, 6th Floor, Boston, MA 02110. The Reporting Person is a citizen of the United States of America. Y The ownership information below represents beneficial ownership of Series A Common Stock of the Issuer as of September 30, 2025, based on (i) 137,345,197 shares of Series A Common Stock outstanding as of September 30, 2025, as reported by the Issuer to the Reporting Person, plus (ii) 31,999,106 shares of Series A Common Stock issuable upon conversion of shares of the Issuer's Series B common stock, par value $0.001 per share (the "Series B Common Stock"), held directly by the Reporting Person, which are deemed outstanding pursuant to SEC Rule 13-3(d)(1)(i) only for purposes of computing the percentage ownership of the Reporting Person. The Reporting Person may be deemed to be the beneficial owner of 32,879,098 shares of Series A Common Stock, which includes: (i) 31,999,106 shares of Series A Common Stock underlying shares of Series B Common Stock held directly by the Reporting Person, (ii) 177,776 shares of Series A Common Stock held by Hodgkins Trust for the benefit of the Reporting Person and the Reporting Person's family, and of which the trustee is an independent institution, and (iii) 702,216 shares of Series A Common Stock held by Hodgkins LLC, of which the Reporting Person serves as manager. 19.4% 32,879,098 0 32,879,098 0 Y N Y Y Y Y Y Hallen Ed /s/ Ed Hallen 11/05/2025