144: Filer Information

Filer CIK

0001634426

Filer CCC

XXXXXXXX

Is this a LIVE or TEST Filing?

LIVE

TEST

Submission Contact Information

Name

Phone

E-Mail Address

144: Issuer Information

Name of Issuer

KLAVIYO, INC

SEC File Number

001-41806

Address of Issuer

125 SUMMER ST, 6TH FLOOR
BOSTON
MASSACHUSETTS
02110

Phone

617-213-1788

Name of Person for Whose Account the Securities are To Be Sold

Summit Partners Growth Equity Fund IX-B, L.P.

See the definition of "person" in paragraph (a) of Rule 144. Information is to be given not only as to the person for whose account the securities are to be sold but also as to all other persons included in that definition. In addition, information shall be given as to sales by all persons whose sales are required by paragraph (e) of Rule 144 to be aggregated with sales for the account of the person filing this notice.

Relationship to Issuer

10% Stockholder

144: Securities Information

Title of the Class of Securities To Be Sold

Name and Address of the Broker

Number of Shares or Other Units To Be Sold

Aggregate Market Value

Number of Shares or Other Units Outstanding

Approximate Date of Sale

Name the Securities Exchange

Series A Common stock, par value $0.001 per share

Goldman Sachs & Co. LLC
200 West Street
New York � NY � 10282

1845131

33876605.16

126937933

08/11/2026

NYSE

Furnish the following information with respect to the acquisition of the securities to be sold and with respect to the payment of all or any part of the purchase price or other consideration therefor:

144: Securities To Be Sold

Title of the Class

Date you Acquired

Nature of Acquisition Transaction

Name of Person from Whom Acquired

Is this a Gift?

Date Donor Acquired

Amount of Securities Acquired

Date of Payment

Nature of Payment *

Series A common stock, par value $0.001 per share

09/20/2023

Pre-IPO Transactions

Klaviyo, Inc.

20086500

09/20/2023

Equity interests in the Issuer were purchased pre-IPO.

* If the securities were purchased and full payment therefor was not made in cash at the time of purchase, explain in the table or in a note thereto the nature of the consideration given. If the consideration consisted of any note or other obligation, or if payment was made in installments describe the arrangement and state when the note or other obligation was discharged in full or the last installment paid.

Furnish the following information as to all securities of the issuer sold during the past 3 months by the person for whose account the securities are to be sold.

144: Securities Sold During The Past 3 Months

Nothing to Report

144: Remarks and Signature

Remarks

Date of Notice

08/11/2026

ATTENTION:

The person for whose account the securities to which this notice relates are to be sold hereby represents by signing this notice that he does not know any material adverse information in regard to the current and prospective operations of the Issuer of the securities to be sold which has not been publicly disclosed. If such person has adopted a written trading plan or given trading instructions to satisfy Rule 10b5-1 under the Exchange Act, by signing the form and indicating the date that the plan was adopted or the instruction given, that person makes such representation as of the plan adoption or instruction date.

Signature

Adam Hennessey