[SCHEDULE 13G/A] Klaviyo, Inc. SEC Filing

Rhea-AI Filing Summary

Summit Partners and several affiliated investment vehicles report collective beneficial ownership of Series A Common Stock of Klaviyo, Inc. representing 30,827,778 Series A Shares issuable in respect of Series B Shares held by the Summit Holders and an aggregate stake of 20.87% of the outstanding Series A shares. The filing lists individual holdings by fund: 18,219,907, 11,376,255, 1,123,429, 95,759 and 12,428 Series A Shares (issuable in respect of equivalent Series B Shares) and states the calculation is based on 116,897,730 Series A Shares reported in the issuer's 10-Q, adjusted for the Series B conversion equivalent.

The disclosure shows shared voting and dispositive power over the reported shares through the Summit ownership and management structure and identifies the investment committee members who may be deemed to have authority. The filing does not assert beneficial ownership beyond the reported amounts and references a joint filing agreement and powers of attorney incorporated by reference.

Positive

• Material institutional backing: Summit Holders collectively report a 20.87% stake in Klaviyo, signaling strong investor interest and support.
• Transparent ownership mapping: The filing discloses specific share counts for each Reporting Person and the managerial chain, improving clarity on voting/dispositive authority.
• Defined decision-makers: The investment committee members (Peter Chung, Scott Collins, Peter Rottier) are identified as potential authorities over the reported securities.

Negative

• Concentrated ownership: A single investor group holds 20.87% of Series A-equivalent shares, a material concentration that could affect governance dynamics.
• No statement of strategic intent: The filing does not disclose any intentions regarding changes in control, governance, or corporate actions, leaving market participants without guidance on future moves.

Insights

Financial Analyst

TL;DR: A single investor group discloses a material, >20% stake in Klaviyo, creating meaningful governance influence without stated change plans.

The Schedule 13G/A quantifies the Summit Holders' economic and voting exposure to Klaviyo through Series B holdings convertible or exercisable into Series A equivalents, totaling 30.8 million shares and representing 20.87% of Series A on the stated base. For investors, this is a material ownership concentration that can affect board-level dynamics and shareholder voting outcomes. The filing documents the internal ownership chain and names the investment committee members with voting/dispositive authority, which clarifies who may direct the stake but does not disclose any strategic intentions or proposals.

Corporate Governance Expert

TL;DR: Summit Partners reports material shared voting/dispositive power over a significant block, a governance-relevant disclosure without operational indications.

The statement details the chain of control from underlying funds to Summit Partners, L.P., and identifies shared voting and dispositive power for the aggregated 30,827,778 Series A-equivalent shares. This level of concentrated ownership (reported as 20.87%) is governance-relevant because it can influence shareholder votes and board composition, yet the filing contains no express plans to form a control group or pursue corporate actions. The inclusion of a joint filing agreement and powers of attorney clarifies procedural coordination among the Reporting Persons.

SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

SCHEDULE 13G

UNDER THE SECURITIES EXCHANGE ACT OF 1934

(Amendment No. 4)

Klaviyo, Inc.

(Name of Issuer)

Series A Common Stock, par value $0.001 per share

(Title of Class of Securities)

49845K101

(CUSIP Number)

06/30/2025

(Date of Event Which Requires Filing of this Statement)

Check the appropriate box to designate the rule pursuant to which this Schedule is filed:

Rule 13d-1(b)

Rule 13d-1(c)

Rule 13d-1(d)

SCHEDULE 13G

CUSIP No. 49845K101

1

Names of Reporting Persons

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

2

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

(a)
(b)

3

Sec Use Only

4

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

9

Aggregate Amount Beneficially Owned by Each Reporting Person

18,219,907.00

10

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

11

Percent of class represented by amount in row (9)

12.33 %

12

Type of Reporting Person (See Instructions)

PN

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

1

Names of Reporting Persons

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

2

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

(a)
(b)

3

Sec Use Only

4

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

9

Aggregate Amount Beneficially Owned by Each Reporting Person

11,376,255.00

10

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

11

Percent of class represented by amount in row (9)

7.70 %

12

Type of Reporting Person (See Instructions)

PN

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

1

Names of Reporting Persons

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

2

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

(a)
(b)

3

Sec Use Only

4

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

9

Aggregate Amount Beneficially Owned by Each Reporting Person

1,123,429.00

10

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

11

Percent of class represented by amount in row (9)

0.76 %

12

Type of Reporting Person (See Instructions)

PN

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

1

Names of Reporting Persons

Summit Investors GE IX/VC IV, LLC

2

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

(a)
(b)

3

Sec Use Only

4

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

9

Aggregate Amount Beneficially Owned by Each Reporting Person

95,759.00

10

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

11

Percent of class represented by amount in row (9)

0.06 %

12

Type of Reporting Person (See Instructions)

OO

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

1

Names of Reporting Persons

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

2

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

(a)
(b)

3

Sec Use Only

4

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

9

Aggregate Amount Beneficially Owned by Each Reporting Person

12,428.00

10

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

11

Percent of class represented by amount in row (9)

0.01 %

12

Type of Reporting Person (See Instructions)

PN

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

1

Names of Reporting Persons

Summit Partners, L.P.

2

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

(a)
(b)

3

Sec Use Only

4

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

9

Aggregate Amount Beneficially Owned by Each Reporting Person

30,827,778.00

10

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

11

Percent of class represented by amount in row (9)

20.87 %

12

Type of Reporting Person (See Instructions)

PN

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

FAQ

How much of Klaviyo (KVYO) do Summit Partners reporting persons own?

The Summit reporting persons collectively report beneficial ownership equivalent to 30,827,778 Series A Shares, representing 20.87% of the Series A class on the stated basis.

Which Summit entities hold shares in KVYO and how many each?

Reported holders and Series A-equivalent amounts are: Fund IX-A 18,219,907, Fund IX-B 11,376,255, Co-Invest (Kiwi) 1,123,429, Fund IX/VC IV 95,759, and Fund IX/VC IV (UK) 12,428.

On what share base is the 20.87% calculated for KVYO?

The percentage is calculated using a reported base of 116,897,730 Series A Shares as disclosed in the issuer's 10-Q, adjusted for the 30,827,778 Series A Shares issuable in respect of Series B Shares held by the Summit Holders.

Does the filing indicate Summit Partners intends to control or change Klaviyo's management?

No. The filing discloses ownership and voting/dispositive authority details but does not state any intent to acquire control or effect management changes.

Who within Summit may exercise voting or dispositive authority over the KVYO shares?

The filing identifies the investment committee of Summit Partners, L.P.—Peter Chung, Scott Collins, and Peter Rottier—as persons who may be deemed to have voting and dispositive authority.