Summary Prospectus by Investment Company - Form 497K

Northwestern Mutual Series Fund, Inc.
Index 400 Stock Portfolio

Supplement Dated October 25, 2024 to the
Summary Prospectus for the Index 400 Stock Portfolio Dated May 1, 2024

The following information supplements the Summary Prospectus for the Index 400 Stock Portfolio of Northwestern Mutual Series Fund, Inc. (the "Fund") dated May 1, 2024 (the "Summary Prospectus"). You should read this Supplement together with the Summary Prospectus.

Portfolio Managers Update - Index 400 Stock Portfolio

Effective on September 24, 2024, Brent Reeder no longer serves as a co-portfolio manager for the Index 400 Stock Portfolio (the "Portfolio"). Keith Carroll has joined Lucy Johnston as co-portfolio managers for the Portfolio. Accordingly, the "Portfolio Managers" information set forth in the Summary Prospectus under the "PORTFOLIO MANAGEMENT" section is hereby deleted and replaced with the following:

"Portfolio Managers: Lucy Johnston, Portfolio Manager, joined Northern Trust in 1997 and has managed the Portfolio since 2021.

Keith Carroll, Portfolio Manager, joined Northern Trust in 2007 and has managed the Portfolio since September 2024."

Please retain this Supplement for future reference.