144 0001634426 XXXXXXXX LIVE 0001835830 Klaviyo, Inc. 001-41806 125 Summer Street, 6th Floor Boston MA 02110 617-213-1788 Summit Partners Growth Equity Fund IX-B, L.P. Owner Series A common stock, par value $0.001 per share Barclays Capital Inc.

745 7th Avenue New York NY 10019

1476104 51279852.96 94683729 06/04/2025 NYSE Series A common stock, par value $0.001 per share 09/20/2023 Pre-IPO transactions. Klaviyo, Inc. N 20086500 09/20/2023 Equity interests in the Issuer were purchased pre-IPO. N Summit Partners Growth Equity Fund IX-B, L.P.

222 Berkeley St 18th FL Boston MA 02116

Series A common stock, par value $0.001 per share 03/14/2025 738052 23617664 Summit Partners Growth Equity Fund IX-B, L.P.

222 Berkeley St 18th FL Boston MA 02116

Series A common stock, par value $0.001 per share 05/19/2025 1476104 48711432 06/04/2025 /s/ Adam Hennessey