SCHEDULE 13G/A 0001991099 XXXXXXXX LIVE 3 Series A common stock, par value $0.001 per share (the "Series A Common Stock") 12/31/2025 0001835830 Klaviyo, Inc. (the "Issuer") 49845K101 125 Summer Street, 6th Floor Boston MA 02110 Rule 13d-1(d) Bialecki Andrew X1 78163529.00 0.00 78163529.00 0.00 78163529.00 N 35.1 IN Klaviyo, Inc. (the "Issuer") 125 Summer Street, 6th Floor, Boston, MA, 02110 This statement is filed on behalf of Andrew Bialecki (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 December 31, 2025, based on (i) 144,262,443 shares of Series A Common Stock outstanding as of December 31, 2025, as reported by the Issuer to the Reporting Person, plus (ii) an aggregate of 78,163,529 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"), beneficially owned 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 78,163,529 shares of Series A Common Stock, which includes: (i) 69,568,889 shares of Series A Common Stock underlying shares of Series B Common Stock held directly by the Reporting Person, (ii) 43,218 shares of Series A Common Stock underlying shares of Series B Common Stock held by the Reporting Person's spouse, (iii) 7,517,410 shares of Series A Common Stock underlying shares of Series B Common Stock held by the Andrew P. Bialecki Grantor Retained Annuity Trust I of 2023, of which the Reporting Person serves as trustee, (iv) 517,006 shares of Series A Common Stock underlying shares of Series B Common Stock held by the Andrew P. Bialecki Irrevocable GST Trust of 2023, of which the Reporting Person's spouse serves as a trustee, and (v) 517,006 shares of Series A Common Stock underlying shares of Series B Common Stock held by the Elizabeth L. Bialecki Irrevocable GST Trust of 2023, of which the Reporting Person serves as a trustee. 35.1% 78,163,529 0 78,163,529 0 Y N Y Y Y Y Y Bialecki Andrew /s/ Andrew Bialecki 02/10/2026