SCHEDULE 13G/A: Klaviyo Co-Founder Bialecki Boosts Stake to 37.7%

Andrew Bialecki, co-founder of Klaviyo, Inc., has reported beneficial ownership of 37.7% of the company's Series A common stock as of September 30, 2025.

Summary

• Andrew Bialecki, a co-founder of Klaviyo, Inc., reported beneficial ownership of 83,050,885 shares of Series A common stock as of September 30, 2025.
• This ownership represents 37.7% of the total Series A common stock class.
• The calculation of the percentage is based on 137,345,197 shares of Series A Common Stock outstanding and 83,050,885 shares of Series A Common Stock issuable upon conversion of Series B common stock beneficially owned by Mr. Bialecki.
• Mr. Bialecki holds sole voting and dispositive power over all 83,050,885 shares.
• The beneficial ownership includes 74,456,245 shares held directly, 43,218 shares held by his spouse, 7,517,410 shares held by the Andrew P. Bialecki Grantor Retained Annuity Trust I of 2023 (where he serves as trustee), 517,006 shares held by the Andrew P. Bialecki Irrevocable GST Trust of 2023 (where his spouse serves as trustee), and 517,006 shares held by the Elizabeth L. Bialecki Irrevocable GST Trust of 2023 (where he serves as trustee).

Sentiment

Score: 7

Explanation: The filing indicates a very high level of beneficial ownership by a co-founder, which is generally a positive signal for investor confidence and management alignment, though it does not provide operational or financial performance details.

Positives

• The significant beneficial ownership by co-founder Andrew Bialecki, representing 37.7% of the Series A common stock, signals strong insider confidence in Klaviyo's long-term prospects and strategic direction.

Future Outlook

This filing is a disclosure of beneficial ownership and does not contain forward-looking statements or guidance regarding the company's future performance or outlook.

Industry Context

High levels of insider ownership, particularly by founders, are often viewed positively by the market as they suggest strong alignment between management and shareholder interests. This can differentiate a company from competitors where insider stakes might be lower, potentially indicating greater commitment to the company's success.

Related Party Transactions

• Beneficial ownership includes shares held through the Andrew P. Bialecki Grantor Retained Annuity Trust I of 2023 and the Elizabeth L. Bialecki Irrevocable GST Trust of 2023, where Andrew Bialecki serves as trustee, and the Andrew P. Bialecki Irrevocable GST Trust of 2023, where his spouse serves as a trustee. These trusts are considered related parties in the context of ownership structure.

Stakeholder Impact

• Shareholders may view the substantial insider ownership as a positive indicator of management's commitment and confidence in the company's long-term value, potentially boosting investor sentiment.

Key Dates

Recommendation

The filing indicates a substantial and consolidated beneficial ownership by co-founder Andrew Bialecki, holding 37.7% of Klaviyo's Series A common stock. This high level of insider ownership, including shares held directly and through various trusts, typically signals strong confidence from key management in the company's long-term prospects. While this filing does not provide operational or financial performance data, the significant insider stake is a positive indicator for investor confidence, supporting a 'hold' recommendation for existing shareholders.

Keywords

Klaviyo, Andrew Bialecki, beneficial ownership, Series A common stock, insider ownership, stock ownership, Schedule 13G