144 0001830877 XXXXXXXX LIVE 0001835830 Klaviyo, Inc. 001-41806 125 Summer Street, 6th Floor Boston MA 02110 617-213-1788 Summit Partners Co-Invest (Kiwi), LP Owner Series A common stock, par value $0.001 per share Barclays Capital Inc.

745 7th Avenue New York NY 10019

145768 4839497.60 116897730 09/08/2025 NYSE Series A common stock, par value $0.001 per share 09/20/2023 Pre-IPO transactions. Klaviyo, Inc. N 1983585 09/20/2023 Equity interests in the Issuer were purchased pre-IPO. Y 09/08/2025 /s/ Adam Hennessey