Summary Prospectus by Investment Company (Form 497K)

MML SERIES INVESTMENT FUND

MML Equity Index Fund

(the "Fund")

Supplement dated December 19, 2025 to the

Prospectus dated April 25, 2025 and the

Summary Prospectus dated April 25, 2025

This supplement provides new and additional information beyond that contained in the Prospectus and Summary Prospectus, and any previous supplements. It should be retained and read in conjunction with the Prospectus and Summary Prospectus, and any previous supplements.

The Fund is updating its diversification policy under the Investment Company Act of 1940, as amended (the "1940 Act"), to reflect that the Fund intends to be diversified in approximately the same proportion as the Fund's underlying index is diversified. The Fund's investment objective is not changing and will continue to be to seek to provide investment results that correspond to the total return performance of publicly traded common stocks in the aggregate as represented by the S&P 500^® Index. The Fund may become "non-diversified," as defined in the 1940 Act, solely as a result of a change in relative market capitalization or index weighting of one or more constituents of the Fund's underlying index. Shareholder approval will not be sought if the Fund crosses from diversified to non-diversified status under such circumstances.

The following information supplements the information found under the heading Principal Investment Strategies in the section titled Investments, Risks, and Performance (beginning on page 13 of the Prospectus):

The Fund intends to be diversified in approximately the same proportion as the Index is diversified. The Fund may become "non-diversified," as defined in the Investment Company Act of 1940, as amended (the "1940 Act"), solely as a result of a change in relative market capitalization or index weighting of one or more constituents of the Index. In these circumstances, the Fund may hold larger positions in a smaller number of issuers than a diversified fund. Shareholder approval will not be sought if the Fund becomes "non-diversified" due solely to a change in the relative market capitalization or index weighting of one or more constituents of the Index.

The following information supplements the information found under the heading Principal Risks in the section titled Investments, Risks, and Performance (beginning on page 14 of the Prospectus):

Non-Diversification Risk. In seeking to track the Index, the Fund may become non-diversified solely as a result of a change in relative market capitalization or index weighting of one or more constituents of the Index. In such circumstances, because the Fund may invest a larger percentage of its assets in a single issuer or in a smaller number of issuers as compared to a diversified fund, the Fund's performance could be closely tied to the value of one issuer or a small number of issuers and could be more volatile than the performance of a diversified fund.

PLEASE RETAIN THIS SUPPLEMENT FOR FUTURE REFERENCE

MMLPRO-25-12

EI-25-02