Sec Form 3 Filing - SUMMIT PARTNERS L P @ Klaviyo, Inc. - 2023-09-20

FORM 3 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

OMB APPROVAL OMB Number: 3235-0287 Expires: November 30, 2011 Estimated average burden hours per response... 0.5

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 (MM/DD/YY) 09/20/2023 (Street) BOSTON, MA02116 4. If Amendment, Date Original Filed (MM/DD/YY) 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 (MM/DD/YY) 2A. Deemed Execution Date, if any (MM/DD/YY) 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

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 (MM/DD/YY) 3A. Deemed Execution Date, if any (MM/DD/YY) 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 (MM/DD/YY) 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 $ 0 ( 1 ) ( 1 ) Series A Common Stock, par value $0.001 per share 54,431,112 ( 2 ) I See footnotes ( 3 ) ( 4 )

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other SUMMIT PARTNERS L P 222 BERKELEY STREET, 18TH FLOOR BOSTON, MA02116 X Summit Partners Growth Equity Fund IX-A, L.P. 222 BERKELEY STREET, 18TH FLOOR BOSTON, MA02116 X Summit Partners Growth Equity Fund IX-B, L.P. 222 BERKELEY STREET, 18TH FLOOR BOSTON, MA02116 X Summit Partners Co-Invest (Kiwi), LP 222 BERKELEY STREET, 18TH FLOOR BOSTON, MA02116 X SUMMIT INVESTORS GE IX/VC IV, LLC 222 BERKELEY STREET, 18TH FLOOR BOSTON, MA02116 X SUMMIT INVESTORS GE IX/VC IV (UK), L.P. 222 BERKELEY STREET, 18TH FLOOR BOSTON, MA02116 X Signatures /s/ Adam Hennessey, as attorney-in-fact for Summit Partners, L.P. 09/20/2023 Signature of Reporting Person Date /s/ Adam Hennessey, as attorney-in-fact for Summit Partners Growth Equity Fund IX-A, L.P. 09/20/2023 Signature of Reporting Person Date /s/ Adam Hennessey, as attorney-in-fact for Summit Partners Growth Equity Fund IX-B, L.P. 09/20/2023 Signature of Reporting Person Date /s/ Adam Hennessey, as attorney-in-fact for Summit Partners Co-Invest (Kiwi), L.P. 09/20/2023 Signature of Reporting Person Date /s/ Adam Hennessey, as attorney-in-fact for Summit Investors GE IX/VC IV, LLC 09/20/2023 Signature of Reporting Person Date /s/ Adam Hennessey, as attorney-in-fact for Summit Investors GE IX/VC IV (UK), L.P. 09/20/2023 Signature of Reporting Person Date

Explanation of Responses:

( 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. ( 2 )Consists of (i) 32,170,006 shares of Series B common stock held by Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A"), (ii) 20,086,500 shares of Series B Common Stock held by Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"), (iii) 1,983,585 shares of Series B Common Stock held by Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"), (iv) 169,076 shares of Series B Common Stock held by Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV"), and (v) 21,945 shares of Series B Common Stock held by Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)"). ( 3 )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 Fund IX-A and 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 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 Fund IX/VC IV and the general partner of Fund IX/VC IV (UK). 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.

Remarks: Exhibit 24 - Power of Attorney

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.

* 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).

Note: File three copies of this Form, one of which must be manually signed. If space is insufficient, see Instruction 6 for procedure.
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 Number.