Statement of Changes in Beneficial Ownership (Form 4)

Ownership Submission

FORM 4 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b). Check this box to indicate that a transaction was made pursuant to a contract, instruction or written plan that is intended to satisfy the affirmative defense conditions of Rule 10b5-1(c). See Instruction 10.

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP OF SECURITIES Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934 or Section 30(h) of the Investment Company Act of 1940

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1. Name and Address of Reporting Person * SUMMIT PARTNERS L P 2. Issuer Name and Ticker or Trading Symbol Klaviyo, Inc. [KVYO] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) _____ Director __X__ 10% Owner _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) 222 BERKELEY STREET, 18TH FLOOR 3. Date of Earliest Transaction (Month/Day/Year) 12/09/2024 (Street) BOSTON, MA 02116 4. If Amendment, Date Original Filed (Month/Day/Year) 6. Individual or Joint/Group Filing (Check Applicable Line) ___ Form filed by One Reporting Person _X_ Form filed by More than One Reporting Person (City) (State) (Zip) Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned

1.Title of Security (Instr. 3) 2. Transaction Date (Month/Day/Year) 2A. Deemed Execution Date, if any (Month/Day/Year) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4) Code V Amount (A) or (D) Price Series A Common Stock, par value $0.001 per share 12/09/2024 C 2,100,000 A (1) 2,100,000 I See footnote(2)(3)(4) Series A Common Stock, par value $0.001 per share 12/09/2024 S 2,100,000 D $39.15 0 I See footnotes(2)(3)(4)

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.
Persons who respond to the collection of information contained in this form are not required to respond unless the form displays a currently valid OMB control number.	SEC 1474 (9-02)

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned
(e.g., puts, calls, warrants, options, convertible securities)

1. Title of Derivative Security (Instr. 3)

2. Conversion or Exercise Price of Derivative Security

3. Transaction Date (Month/Day/Year)

3A. Deemed Execution Date, if any (Month/Day/Year)

4. Transaction Code (Instr. 8)

5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4, and 5)

6. Date Exercisable and Expiration Date (Month/Day/Year)

7. Title and Amount of Underlying Securities (Instr. 3 and 4)

8. Price of Derivative Security (Instr. 5)

9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4)

10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4)

11. Nature of Indirect Beneficial Ownership (Instr. 4)

Code

V

(A)

(D)

Date Exercisable

Expiration Date

Title

Amount or Number of Shares

Series B Common Stock, par value $0.001 per share

(1)

12/09/2024

C

2,100,000

(1)

(1)

Series A Common Stock, par value $0.001 per share

2,100,000

$ 0

40,827,778(5)

I

See footnote(2)(3)(4)

Reporting Owners

Reporting Owner Name / Address	Relationships
Director	10% Owner	Officer	Other
SUMMIT PARTNERS L P 222 BERKELEY STREET, 18TH FLOOR BOSTON, MA 02116		X
Summit Partners Growth Equity Fund IX-A, L.P. 222 BERKELEY STREET, 18TH FLOOR BOSTON, MA 02116		X
Summit Partners Growth Equity Fund IX-B, L.P. 222 BERKELEY STREET, 18TH FLOOR BOSTON, MA 02116		X
Summit Partners Co-Invest (Kiwi), LP 222 BERKELEY STREET, 18TH FLOOR BOSTON, MA 02116		X
SUMMIT INVESTORS GE IX/VC IV, LLC 222 BERKELEY STREET, 18TH FLOOR BOSTON, MA 02116		X
SUMMIT INVESTORS GE IX/VC IV (UK), L.P. 222 BERKELEY STREET, 18TH FLOOR BOSTON, MA 02116		X

Signatures

/s/ Adam Hennessey, as attorney-in-fact for Summit Partners, L.P.	12/11/2024
**Signature of Reporting Person	Date
/s/ Adam Hennessey, as attorney-in-fact for Summit Investors GE IX/VC IV, LLC	12/11/2024
**Signature of Reporting Person	Date
/s/ Adam Hennessey, as attorney-in-fact for Summit Partners Co-Invest (Kiwi), L.P.	12/11/2024
**Signature of Reporting Person	Date
/s/ Adam Hennessey, as attorney-in-fact for Summit Partners Growth Equity Fund IX-A, L.P.	12/11/2024
**Signature of Reporting Person	Date
/s/ Adam Hennessey, as attorney-in-fact for Summit Investors GE IX/VC IV (UK), L.P.	12/11/2024
**Signature of Reporting Person	Date
/s/ Adam Hennessey, as attorney-in-fact for Summit Partners Growth Equity Fund IX-B, L.P.	12/11/2024
**Signature of Reporting Person	Date

Explanation of Responses:

*	If the form is filed by more than one reporting person, see Instruction 4(b)(v).
**	Intentional misstatements or omissions of facts constitute Federal Criminal Violations. See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).
(1)	The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation. On December 9, 2024, the reporting persons directed the sale of an aggregate of 2,100,000 shares of their Series B Common Stock, resulting in the automatic conversion of the shares into Series A Common Stock upon execution of the sale.
(2)	Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, LP, which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") and Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"). Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"). Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") and the general partner of Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)").
(3)	(continued from footnote 2) Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the Series B Common Stock held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P.
(4)	The investment committee of Summit Partners, L.P., is currently composed of Peter Y. Chung, Scott C. Collins, and Peter L. Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the shares, except to the extent of such person's or entity's pecuniary interest therein and the filing of this statement shall not be deemed an admission that any of the foregoing entities or persons are, for purposes of Section 16 of the Act or otherwise, the beneficial owner of any equity securities covered by this statement.
(5)	Consists of (i) 24,130,130 shares of Series B Common Stock held by Fund IX-A, (ii) 15,066,515 shares of Series B Common Stock held by Fund IX-B, (iii) 1,487,851 shares of Series B Common Stock held by Kiwi Co-Invest Fund, (iv) 126,822 shares of Series B Common Stock held by Fund IX/VC IV, and (v) 16,460 shares of Series B Common Stock held by Fund IX/VC IV (UK).

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