SCHEDULE 13G

CUSIP No.

49845K101

Names of Reporting Persons

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

Check the appropriate box if a member of a Group (see instructions)

(a)

(b)

Sec Use Only

Citizenship or Place of Organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

5	Sole Voting Power 0.00
6	Shared Voting Power 18,219,907.00
7	Sole Dispositive Power 0.00
8	Shared Dispositive Power 18,219,907.00

Aggregate Amount Beneficially Owned by Each Reporting Person

18,219,907.00

Check box if the aggregate amount in row (9) excludes certain shares (See Instructions)

Checkbox not checked

Percent of class represented by amount in row (9)

12.33 %

Type of Reporting Person (See Instructions)

Comment for Type of Reporting Person: Represents 18,219,907 shares of Series A Common Stock ("Series A Shares") issuable in respect of 18,219,907 shares of Series B Common Stock ("Series B Shares"). Calculated based upon 116,897,730 Series A Shares outstanding as of July 31, 2025 as reported on the Issuer's Quarterly Report on Form 10-Q (the "10-Q") filed with the Securities and Exchange Commission ("SEC") on August 5, 2025, as increased by an aggregate of 30,827,778 Series A Shares issuable in respect of the 30,827,778 Series B Shares held by the Summit Holders (as defined in Item 2).

SCHEDULE 13G

CUSIP No.

49845K101

Names of Reporting Persons

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

Check the appropriate box if a member of a Group (see instructions)

(a)

(b)

Sec Use Only

Citizenship or Place of Organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

5	Sole Voting Power 0.00
6	Shared Voting Power 11,376,255.00
7	Sole Dispositive Power 0.00
8	Shared Dispositive Power 11,376,255.00

Aggregate Amount Beneficially Owned by Each Reporting Person

11,376,255.00

Check box if the aggregate amount in row (9) excludes certain shares (See Instructions)

Checkbox not checked

Percent of class represented by amount in row (9)

7.70 %

Type of Reporting Person (See Instructions)

Comment for Type of Reporting Person: Represents 11,376,255 Series A Shares issuable in respect of 11,376,255 Series B Shares. Calculated based upon 116,897,730 Series A Shares outstanding as of July 31, 2025 as reported on the Issuer's 10-Q filed with the SEC on August 5, 2025, as increased by an aggregate of 30,827,778 Series A Shares issuable in respect of the 30,827,778 Series B Shares held by the Summit Holders (as defined in Item 2).

SCHEDULE 13G

CUSIP No.

49845K101

Names of Reporting Persons

Summit Partners Co-Invest (Kiwi), L.P.

Check the appropriate box if a member of a Group (see instructions)

(a)

(b)

Sec Use Only

Citizenship or Place of Organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

5	Sole Voting Power 0.00
6	Shared Voting Power 1,123,429.00
7	Sole Dispositive Power 0.00
8	Shared Dispositive Power 1,123,429.00

Aggregate Amount Beneficially Owned by Each Reporting Person

1,123,429.00

Check box if the aggregate amount in row (9) excludes certain shares (See Instructions)

Checkbox not checked

Percent of class represented by amount in row (9)

0.76 %

Type of Reporting Person (See Instructions)

Comment for Type of Reporting Person: Represents 1,123,429 Series A Shares issuable in respect of 1,123,429 Series B Shares. Calculated based upon 116,897,730 Series A Shares outstanding as of July 31, 2025 as reported on the Issuer's 10-Q filed with the SEC on August 5, 2025, as increased by an aggregate of 30,827,778 Series A Shares issuable in respect of the 30,827,778 Series B Shares held by the Summit Holders (as defined in Item 2).

SCHEDULE 13G

CUSIP No.

49845K101

Names of Reporting Persons

Summit Investors GE IX/VC IV, LLC

Check the appropriate box if a member of a Group (see instructions)

(a)

(b)

Sec Use Only

Citizenship or Place of Organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

5	Sole Voting Power 0.00
6	Shared Voting Power 95,759.00
7	Sole Dispositive Power 0.00
8	Shared Dispositive Power 95,759.00

Aggregate Amount Beneficially Owned by Each Reporting Person

95,759.00

Check box if the aggregate amount in row (9) excludes certain shares (See Instructions)

Checkbox not checked

Percent of class represented by amount in row (9)

0.06 %

Type of Reporting Person (See Instructions)

Comment for Type of Reporting Person: Represents 95,759 Series A Shares issuable in respect of 95,759 Series B Shares. Calculated based upon 116,897,730 Series A Shares outstanding as of July 31, 2025 as reported on the Issuer's 10-Q filed with the SEC on August 5, 2025, as increased by an aggregate of 30,827,778 Series A Shares issuable in respect of the 30,827,778 Series B Shares held by the Summit Holders (as defined in Item 2).

SCHEDULE 13G

CUSIP No.

49845K101

Names of Reporting Persons

Summit Investors GE IX/VC IV (UK), L.P.

Check the appropriate box if a member of a Group (see instructions)

(a)

(b)

Sec Use Only

Citizenship or Place of Organization

CAYMAN ISLANDS

Number of Shares Beneficially Owned by Each Reporting Person With:

5	Sole Voting Power 0.00
6	Shared Voting Power 12,428.00
7	Sole Dispositive Power 0.00
8	Shared Dispositive Power 12,428.00

Aggregate Amount Beneficially Owned by Each Reporting Person

12,428.00

Check box if the aggregate amount in row (9) excludes certain shares (See Instructions)

Checkbox not checked

Percent of class represented by amount in row (9)

0.01 %

Type of Reporting Person (See Instructions)

Comment for Type of Reporting Person: Represents 12,428 Series A Shares issuable in respect of 12,428 Series B Shares. Calculated based upon 116,897,730 Series A Shares outstanding as of July 31, 2025 as reported on the Issuer's 10-Q filed with the SEC on August 5, 2025, as increased by an aggregate of 30,827,778 Series A Shares issuable in respect of the 30,827,778 Series B Shares held by the Summit Holders (as defined in Item 2).

SCHEDULE 13G

CUSIP No.

49845K101

Names of Reporting Persons

Summit Partners, L.P.

Check the appropriate box if a member of a Group (see instructions)

(a)

(b)

Sec Use Only

Citizenship or Place of Organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

5	Sole Voting Power 0.00
6	Shared Voting Power 30,827,778.00
7	Sole Dispositive Power 0.00
8	Shared Dispositive Power 30,827,778.00

Aggregate Amount Beneficially Owned by Each Reporting Person

30,827,778.00

Check box if the aggregate amount in row (9) excludes certain shares (See Instructions)

Checkbox not checked

Percent of class represented by amount in row (9)

20.87 %

Type of Reporting Person (See Instructions)

Comment for Type of Reporting Person: Represents 30,827,778 Series A Shares issuable in respect of 30,827,778 Series B Shares. Calculated based upon 116,897,730 Series A Shares outstanding as of July 31, 2025 as reported on the Issuer's 10-Q filed with the SEC on August 5, 2025, as increased by an aggregate of 30,827,778 Series A Shares issuable in respect of the 30,827,778 Series B Shares held by the Summit Holders (as defined in Item 2).

SCHEDULE 13G

Item 1.

(a)

Name of issuer:

Klaviyo, Inc.

(b)

Address of issuer's principal executive offices:

125 Summer Street, Floor 6, Boston, MA 02110

Item 2.

(a)

Name of person filing:

This statement is filed by the entities and persons listed below, all of whom together are referred to herein as the "Reporting Persons": (i) Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") (ii) Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B") (iii) Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund") (iv) Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") (v) Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)," and together with each of the foregoing, the "Summit Holders") (vi) Summit Partners, L.P.

(b)

Address or principal business office or, if none, residence:

222 Berkeley Street, 18th Floor Boston, MA 02116

(c)

Citizenship:

See responses to Item 4 on each cover page.

(d)

Title of class of securities:

Series A Common Stock, par value $0.001 per share

(e)

CUSIP No.:

49845K101

Item 3.

If this statement is filed pursuant to §§ 240.13d-1(b) or 240.13d-2(b) or (c), check whether the person filing is a:

(a)

Broker or dealer registered under section 15 of the Act (15 U.S.C. 78o);

(b)

Bank as defined in section 3(a)(6) of the Act (15 U.S.C. 78c);

(c)

Insurance company as defined in section 3(a)(19) of the Act (15 U.S.C. 78c);

(d)

Investment company registered under section 8 of the Investment Company Act of 1940 (15 U.S.C. 80a-8);

(e)

An investment adviser in accordance with § 240.13d-1(b)(1)(ii)(E);

(f)

An employee benefit plan or endowment fund in accordance with § 240.13d-1(b)(1)(ii)(F);

(g)

A parent holding company or control person in accordance with § 240.13d-1(b)(1)(ii)(G);

(h)

A savings associations as defined in Section 3(b) of the Federal Deposit Insurance Act (12 U.S.C. 1813);

(i)

A church plan that is excluded from the definition of an investment company under section 3(c)(14) of the Investment Company Act of 1940 (15 U.S.C. 80a-3);

(j)

A non-U.S. institution in accordance with § 240.13d-1(b)(1)(ii)(J). If filing as a non-U.S. institution in accordance with § 240.13d-1(b)(1)(ii)(J),
please specify the type of institution:

(k)

Group, in accordance with Rule 240.13d-1(b)(1)(ii)(K).

Item 4.

Ownership

(a)

Amount beneficially owned:

See responses to Item 9 on each cover page. The aggregate Series A Shares beneficially owned by the Reporting Persons and reflected in this statement represents 20.87% of the outstanding Series A Shares. Such calculation is based upon 116,897,730 Series A Shares outstanding as of July 31, 2025 as reported on the Issuer's 10-Q filed with the SEC on August 5, 2025, as increased by an aggregate of 30,827,778 Series A Shares issuable in respect of the 30,827,778 Series B Shares held by the Summit Holders. In this regard, the reported securities are directly held by Fund IX-A (18,219,907 Series A Shares issuable in respect of 18,219,907 Series B Shares), Fund IX-B (11,376,255 Series A Shares issuable in respect of 11,376,255 Series B Shares), Kiwi Co-Invest Fund (1,123,429 Series A Shares issuable in respect of 1,123,429 Series B Shares), Fund IX/VC IV (95,759 Series A Shares issuable in respect of 95,759 Series B Shares), and Fund IX/VC IV (UK) (12,428 Series A Shares issuable in respect of 12,428 Series B Shares). 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 Shares held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P. The investment committee of Summit Partners, L.P. is currently composed of Peter Chung, Scott Collins and Peter 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 reported securities (except to the extent such securities are directly held by such entity). The filing of this statement shall not be construed as an admission that the Reporting Persons or any of the foregoing are the beneficial owners of any of the securities covered by this statement.

(b)

Percent of class:

See responses to Item 11 on each cover page

(c)

Number of shares as to which the person has:

(i) Sole power to vote or to direct the vote:

See responses to Item 5 on each cover page.

(ii) Shared power to vote or to direct the vote:

See responses to Item 6 on each cover page.

(iii) Sole power to dispose or to direct the disposition of:

See responses to Item 7 on each cover page.

(iv) Shared power to dispose or to direct the disposition of:

See responses to Item 8 on each cover page.

Item 5.

Ownership of 5 Percent or Less of a Class.

Item 6.

Ownership of more than 5 Percent on Behalf of Another Person.

Not Applicable

Item 7.

Identification and Classification of the Subsidiary Which Acquired the Security Being Reported on by the Parent Holding Company or Control Person.

Not Applicable

Item 8.

Identification and Classification of Members of the Group.

Not Applicable

Item 9.

Notice of Dissolution of Group.

Not Applicable

Item 10.

Certifications:

Not Applicable

SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

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

Signature:	/s/ Adam H. Hennessey, as POA
Name/Title:	Adam H. Hennessey, as Power of Attorney
Date:	08/08/2025

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

Signature:	/s/ Adam H. Hennessey, as POA
Name/Title:	Adam H. Hennessey, as Power of Attorney
Date:	08/08/2025

Summit Partners Co-Invest (Kiwi), L.P.

Signature:	/s/ Adam H. Hennessey, as POA
Name/Title:	Adam H. Hennessey, as Power of Attorney
Date:	08/08/2025

Summit Investors GE IX/VC IV, LLC

Signature:	/s/ Adam H. Hennessey, as POA
Name/Title:	Adam H. Hennessey, as Power of Attorney
Date:	08/08/2025

Summit Investors GE IX/VC IV (UK), L.P.

Signature:	/s/ Adam H. Hennessey, as POA
Name/Title:	Adam H. Hennessey, as Power of Attorney
Date:	08/08/2025

Summit Partners, L.P.

Signature:	/s/ Adam H. Hennessey, as POA
Name/Title:	Adam H. Hennessey, as Power of Attorney
Date:	08/08/2025

Comments accompanying signature: Exhibit A Joint Filing Agreement, dated as of January 31, 2024, incorporated by reference to the Schedule 13G filed by the Reporting Persons on January 31, 2024 Exhibit B Powers of Attorney, incorporated herein by reference to of the statement on Schedule 13G filed by the Reporting Persons on January 31, 2024