Domestic Equity Portfolio

SUMMARY PROSPECTUS

MAY 1, 2026

Before you invest, you may want to review the Portfolio’s prospectus, which contains more information about the Portfolio and its risks. You can find the Portfolio’s prospectus, reports to shareholders, and other information about the Portfolio online at www.nmseriesfund.com. You can also get this information at no cost by calling (866) 910-1232 or by sending an e-mail request to sfprospectus@northwesternmutual.com. The current prospectus and statement of additional information, each dated May 1, 2026, along with the Portfolio’s most recent annual report dated December 31, 2025, are incorporated by reference into this Summary Prospectus. The Portfolio’s statement of additional information and annual report may be obtained, free of charge, in the same manner as the prospectus.

The Securities and Exchange Commission has not approved or disapproved these securities or passed upon the adequacy of this prospectus. Any representation to the contrary is a criminal offense.

INVESTMENT OBJECTIVE

The investment objective of the Portfolio is long-term growth of capital and income.

FEES AND EXPENSES OF THE PORTFOLIO

The table below describes the fees and expenses that you may pay when you buy, hold, and sell interests in a separate account that invests in shares of the Portfolio as a result of your purchase of a variable annuity contract or variable life insurance policy. The fees and expenses shown in the table and Example do not reflect fees and expenses separately charged by variable annuity contracts or variable life insurance policies. If the fees and expenses separately charged by variable annuity contracts and variable life insurance policies were included, the fees and expenses shown in the table and the Example would be higher.

Shareholder Fees

(fees paid directly from your investment)

N/A

Annual Portfolio Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)
Management Fee	0.52%
Distribution and Service (12b-1) Fees	None
Other Expenses	0.02%
Total Annual Portfolio Operating Expenses	0.54%
Fee Waiver(1)	(0.04)%
Total Annual Portfolio Operating Expenses After Fee Waiver(1)	0.50%

(1)

The Portfolio's investment adviser has contractually agreed to waive a portion of its management fee. This contractual agreement will continue through at least April 30, 2027 and may not be terminated prior to that date without action by the Board of Directors.

Example

This Example is intended to help you compare the cost of investing in the Portfolio with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Portfolio for the time periods indicated and then redeem or hold all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Portfolio’s operating expenses remain the same. The Example reflects adjustments made to the Portfolio’s operating expenses due to the fee waiver agreement with the investment adviser for the first year only. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

1 Year	3 Years	5 Years	10 Years
$51	$169	$298	$673

Portfolio Turnover

The Portfolio pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover may indicate higher transaction costs. These costs, which are not reflected in Annual Portfolio Operating Expenses or in the Example, affect the Portfolio’s performance. During the most recent fiscal year, the Portfolio’s portfolio turnover rate was 105% of the average value of its portfolio.

PRINCIPAL INVESTMENT STRATEGIES

Normally, the Portfolio invests at least 80% of net assets (plus any borrowings for investment purposes) in equity securities of U.S. issuers. An issuer of a security will be deemed to be a “U.S. issuer” if: (i) the principal trading market for its securities is the United States; (ii) it is organized under the laws of, or has principal offices in, the United States; or (iii) it derives 50% or more of its total revenue from goods or services produced or sold in U.S.

The Portfolio primarily invests in common stocks that offer potential for growth of capital, current income, or both. The Portfolio mainly invests in large- and mid-capitalization companies. Typically, the Portfolio seeks securities the adviser believes are currently undervalued by the market. The adviser may consider, among other factors, a company’s valuation, financial strength, growth potential, competitive position in its industry, projected future earnings, cash flows and dividends when deciding whether to buy or sell investments.

While most assets will be invested in U.S. equity securities, in keeping with the Portfolio’s investment objective, it may also invest in

NMSF-SP1Northwestern Mutual Series Fund, Inc.

Domestic Equity Portfolio – Summary

American Depositary Receipts (ADRs) and foreign securities (up to 10% of net assets). The Portfolio may also utilize futures for cash management purposes and forwards to hedge foreign currency exposure.

The Portfolio’s sector exposure relative to its benchmark is driven by the adviser’s stock selection process, and, as a result, the Portfolio may at times have sector weightings which vary from the sector weightings of the benchmark.

The Portfolio may sell a security if it no longer believes the security will contribute to meeting the investment objective of the Portfolio.

PRINCIPAL RISKS

Portfolio shares will rise and fall in value and there is a risk you could lose money by investing in the Portfolio. There can be no assurance that the Portfolio will achieve its objective. The main risks of investing in this Portfolio are identified below.

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Active Management Risk – The adviser’s investment strategies and techniques may not perform as expected which could cause the Portfolio to underperform other mutual funds or lose money.

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ADR Risk – ADRs are receipts representing ownership of shares of a foreign issuer held by a U.S. bank or similar financial institution that entitle the holder to dividends and capital gains on the underlying foreign shares. ADRs are alternatives to directly purchasing the underlying foreign securities in their national markets and currencies. They are subject to many of the risks associated with direct investments in foreign securities, such as currency risk, political and economic risk and market risk, because their values depend on the performance of the non-dollar denominated underlying foreign securities. The Portfolio is also subject to fees and the credit risk of the financial institution holding the ADRs. ADRs may be “sponsored,” meaning that they are implemented by a financial institution in collaboration with the issuing foreign company, or “unsponsored,” meaning that the financial institution created the instrument without the sponsorship or direct involvement of the foreign company. Differing registration requirements apply to each type of ADR. ADRs may transact on exchanges or on over-the-counter markets (“OTC”). Conducting transactions in OTC markets may result in higher costs, a lack of pricing transparency and lower liquidity when compared with exchange-based transactions. Risks associated with different ADR types will vary, based upon differences in registration, reporting and disclosure requirements that apply to such ADRs and the characteristics of the market in which transactions for the particular ADR are conducted.

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Derivatives Risk – The value of a derivative generally depends upon, or is derived from, an underlying asset, reference rate or index. The primary risks associated with the Portfolio’s use of derivatives are the risk that changes in the value of the derivatives may not correlate as intended with the underlying asset, rate or index, the risk of adverse price movements in the market, the risk that the counterparty to a derivatives transaction fails to make the required payment, and the risk of missed opportunities in other investments. Certain derivatives involve leverage, which could cause the Portfolio to lose more than the principal amount invested. Other risks include management, market, interest rate, and liquidity risks. The Portfolio’s purchase of forwards and futures contracts may involve risks related to imperfect correlation between the prices of such instruments and the price of the underlying asset, as well as leverage, liquidity and volatility risks. In addition, the purchase of forwards also involves counterparty credit risk as well as heightened market risk.

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Equity Securities Risk – The value of equity securities, such as common stocks, could decline if the financial condition of the companies the Portfolio is invested in declines or if overall market and economic conditions deteriorate. Equity securities generally have greater price volatility than fixed income securities.

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Foreign Investing Risk – Investing in foreign securities may subject the Portfolio to more rapid and extreme changes in value or more losses than a fund that invests exclusively in U.S. securities. This risk is due to potentially smaller markets, differing reporting, accounting and auditing standards, and nationalization, expropriation or confiscatory taxation, currency blockage, political and economic conditions, or diplomatic developments. Foreign securities may be adversely affected by decreases in foreign currency values relative to the U.S. dollar and may be less liquid, more volatile, and harder to value than U.S. securities. The Portfolio’s investments in emerging markets heighten these risks due to a lack of established legal, political, business and social frameworks to support securities markets.

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Investment Style Risk – A portfolio managed using a value style of investing, such as the Portfolio, may underperform when the market does not favor the particular style used by the Portfolio. Different investment styles tend to shift in and out of favor, depending on market conditions and investor sentiment.

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Large Cap Company Risk – Investing in large cap stocks could cause the Portfolio to underperform in markets favoring faster growing companies. Large cap stocks tend to be more mature with fewer opportunities to grow and may not have the same growth potential as stocks with smaller capitalizations.

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Market Risk – The risk that the market price of securities owned by the Portfolio may go up or down, sometimes rapidly or unpredictably. The value of a security may decline due to changes in general market conditions, economic trends or events that are not specifically related to the issuer of the security, or factors that affect a particular issuer or issuers, exchange, country, group of countries, region, market, industry, group of industries, sector or asset class. Global economies and financial markets are increasingly interconnected, which magnifies the potential that conditions in one country or region might adversely impact issuers in, or foreign exchange rates with, a different country or region. Geopolitical and other events, including war, terrorism,

NMSF-SP2Northwestern Mutual Series Fund, Inc. Prospectus

Domestic Equity Portfolio – Summary

economic uncertainty, trade disputes, tariffs, public health crises (such as epidemics and pandemics), and related events have led, and in the future may lead, to increased market volatility, which may disrupt U.S. and world economies and markets and may have significant adverse direct or indirect effects on the Portfolio and its investments.

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Mid Cap Company Risk – Investing in mid cap stocks may cause greater risk of loss and price fluctuation than investing in stocks of larger cap companies due to a more limited track record, narrower product markets, more limited resources and less liquid trading markets. These stocks may be more volatile and more difficult to buy and sell than stocks with larger capitalizations.

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Sector Focus Risk – To the extent the Portfolio invests a relatively high percentage of its assets in a particular sector, it will have greater exposure to the risks associated with that sector, including the risk that the securities of companies within the sector will underperform due to adverse economic conditions, regulatory or legislative changes, or increased competition affecting the sector. To the extent the Portfolio is underweight in other sectors, the Portfolio risks missing out on advances in those sectors.

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Underlying Portfolio Risk – The Portfolio may serve as an investment option, or “Underlying Portfolio,” for other portfolios of Northwestern Mutual Series Fund, Inc. that are managed as “fund of funds.” As a result, from time to time, the Portfolio may experience relatively large investments or redemptions from those other portfolios and could be required to invest cash or sell securities at a time when it is not advantageous to do so.

NMSF-SP3Northwestern Mutual Series Fund, Inc. Prospectus

Domestic Equity Portfolio – Summary

PERFORMANCE

The following bar chart illustrates the risks of investing in the Portfolio by showing how the performance of the Portfolio has varied from year to year. The table to the right of the bar chart shows the Portfolio’s average annual total return over certain time periods and compares the Portfolio’s returns with those of an index that has characteristics relevant to the Portfolio’s investment strategy (Strategy Index). The table also shows the Portfolio’s returns against an index that represents the overall securities market (Broad-Based Index), which the Portfolio has added to comply with new regulatory requirements. Prior to July 31, 2025, the Portfolio was managed by a different sub-adviser. Performance shown may have been different if the current strategy, and the current sub-adviser, had been in place during the periods shown. Returns are based on past results and are not an indication of future performance. Neither the bar chart nor the table reflects the fees and expenses separately charged by the variable annuity contract or variable life insurance policy separate account that invests in the Portfolio and returns would be lower if those fees and expenses were reflected.

	Quarter/Year	Return
Best Quarter	2nd quarter, 2020	15.36%
Worst Quarter	1st quarter, 2020	-26.34%

Average Annual Total Return
(for periods ended December 31, 2025)

		1 Yr	5 Yr	10 Yr
Portfolio:	Domestic Equity Portfolio	14.43%	8.63%	8.87%
Strategy Index:	Russell 1000® Value Index (reflects no deduction for fees, expenses or taxes)	15.91%	11.33%	10.53%
Broad-Based Index:	S&P 500® Index (reflects no deduction for fees, expenses or taxes)	17.88%	14.42%	14.82%

PORTFOLIO MANAGEMENT

Investment Adviser: Mason Street Advisors, LLC
Sub-Adviser: Putnam Investment Management, LLC (Putnam)
Portfolio Managers: Darren Jaroch, CFA, Portfolio Manager, has been with Putnam since 1999, and has managed the Portfolio since 2025.
Lauren DeMore, CFA, Portfolio Manager, has been with Putnam since 2006, and has managed the Portfolio since 2025.

TAX INFORMATION

Shares of the Portfolio are offered only for funding variable annuity contracts and variable life insurance policies offered by The Northwestern Mutual Life Insurance Company through separate accounts. Insurance company separate accounts generally do not pay tax on dividends or capital gain distributions. Investors in variable annuity contracts and variable life insurance policies should refer to the prospectuses for the variable products for a discussion of the tax considerations that affect the insurance company and its separate accounts and the tax consequences to investors of owning such products.

COMPENSATION TO BROKER-DEALERS AND OTHER FINANCIAL INTERMEDIARIES

Neither the Portfolio nor any related companies pay compensation to broker-dealers or other financial intermediaries for the sale of Portfolio shares or related services. Investors in variable annuity contracts and variable life insurance policies should refer to the prospectuses for the variable products for important information about compensation paid to financial intermediaries for sales of variable annuity contracts and variable life insurance policies.

NMSF-SP4Northwestern Mutual Series Fund, Inc. Prospectus