Active/Passive Very Aggressive Portfolio

Active/Passive Very Aggressive Portfolio (formerly Active/Passive All 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.

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	0.15%
Distribution and Service (12b-1) Fees	None
Other Expenses	0.30%
Acquired Fund Fees and Expenses	0.36%
Total Annual Portfolio Operating Expenses(1)	0.81%
Fee Waiver and Expense Reimbursement(2)	(0.18)%
Total Annual Portfolio Operating Expenses After Fee Waiver and Expense Reimbursement(1),(2)	0.63%

(1)

Includes fees and expenses incurred indirectly by the Portfolio as a result of investments in other investment companies (Acquired Fund Fees and Expenses). The operating expenses of the Portfolio reflected in the Portfolio's most recent annual report and Financial Highlights do not include the Acquired Fund Fees and Expenses.

(2)

The Portfolio's investment adviser has contractually agreed to waive and/or reimburse fees or expenses in order to limit Total Annual Portfolio Operating Expenses After Fee Waiver and/or Expense Reimbursement (excluding taxes, brokerage, other investment-related costs, interest and dividend expenses and charges, acquired fund fees and expenses and such non-recurring and extra ordinary expenses as they may arise) to an annual rate of 0.27% of the Portfolio's average net assets. These contractual agreements 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 and expense reimbursement agreements 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
$64	$241	$432	$985

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 period, the Portfolio’s portfolio turnover rate was 25% of the average value of its portfolio.

NMSF-SP1Northwestern Mutual Series Fund, Inc.

Active/Passive Very Aggressive Portfolio (formerly Active/Passive All Equity Portfolio) – Summary

PRINCIPAL INVESTMENT STRATEGIES

Investing in multiple investment asset classes, the Portfolio attempts to capitalize on the variation in return potential produced by the interaction of changing financial markets and economic conditions while maintaining a balance over time between investment opportunities and their associated potential risks by following a flexible policy of allocating assets.

The Portfolio is one of the Northwestern Mutual Series Fund, Inc. Allocation Portfolios (Active/Passive Allocation Suite) and operates primarily as a “fund of funds.” The Portfolio invests in a combination of both actively managed and passive (i.e., index-tracking) underlying Portfolios of Northwestern Mutual Series Fund, Inc. (each, an “Underlying Portfolio”) and exchange-traded funds (“ETFs”) which are not portfolios of Northwestern Mutual Series Fund, Inc.

As a “very aggressive” Portfolio, the adviser will allocate the Portfolio’s assets to a combination of underlying funds that is expected to have the most aggressive investment risk relative to the other Portfolios of the Active/Passive Allocation Suite. The Portfolio is designed primarily for investors with a higher risk tolerance consistent with a portfolio that holds equity investments in pursuit of long-term growth of capital. The Portfolio has a target asset allocation as indicated below:

	Equity and Other* Exposure	Fixed Income or Debt Exposure	Cash Equivalents
Active/Passive Very Aggressive	80 – 100%	0 – 10%	0 – 10%

*Other exposure includes REIT and commodity-related investments.

These foregoing percentage target ranges are not intended to establish minimum and maximum limits. The adviser may deviate from the asset class target ranges in pursuit of total return and in response to changing market and economic conditions, and, may invest a greater or lesser percentage in any strategic asset class component when the adviser deems it favorable to do so in order to achieve the Portfolio’s investment objective.

Equity and Other Exposure: The Portfolio may obtain equity exposure by investing in one or more Underlying Portfolios or ETFs, which may hold a wide range of equity securities including small, mid and large cap U.S. and non-U.S. stocks. Equity securities could include common and preferred stocks, securities convertible into stocks and depositary receipts for those securities. “Other” exposure includes both real estate investment trusts (“REITs”) and commodities. The Portfolio may obtain exposure to REITs by investing in ETFs that hold REITs. The Portfolio may obtain commodity exposure through the purchase of swaps on physical commodities or commodity indices, including those which are traded in over-the-counter markets (“OTC”), and by investing in ETFs that provide exposure to commodities markets.

Cash Equivalents: The cash equivalent portion of the Portfolio may include, but is not limited to, investments in Underlying Portfolios or ETFs that hold debt securities issued or guaranteed by the U.S. government or its agencies or instrumentalities as well as commercial paper, banker’s acceptances, certificates of deposit and time deposits. In order to enhance short duration returns, the adviser may purchase ultra-short bond ETFs.

The adviser considers a number of factors when making purchase and sales decisions with respect to the Underlying Portfolios and ETFs. With respect to the equity Underlying Portfolios and ETFs, the adviser considers their investment focus on small, mid or large market capitalizations, domestic or foreign investments, whether the Underlying Portfolio or ETF is diversified or non-diversified and whether it employs a particular style of investing, among other characteristics. The adviser regularly reviews and adjusts the allocation among the Underlying Portfolios and ETFs to favor investments in those Underlying Portfolios and ETFs that the adviser believes provide the most favorable position for achieving the Portfolio’s investment objective. In connection with the allocation process, the Portfolio may invest more than 25% of its assets in one Underlying Portfolio or one ETF.

An Underlying Portfolio or ETF may invest a large percentage of its assets in a single issuer, security, market or sector (or a limited group thereof) or in the case of an international Underlying Portfolio or ETF, may invest in emerging markets, a small number of countries or a particular geographic region.

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 Portfolio bears all of the risks associated with the investment strategies used by the Underlying Portfolios and ETFs and other securities in which it invests. Except as otherwise stated, references in this section to the “Portfolio” may relate to the Portfolio, one or more Underlying Portfolios, or both. 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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Affiliated Portfolio Risk – In managing the Portfolio, the adviser has the authority to select, and allocate among, Underlying Portfolios. The adviser may be subject to potential conflicts of interest in selecting Underlying Portfolios because the fees paid to it by some Underlying Portfolios are higher than the fees paid by other Underlying Portfolios. Moreover, a situation could occur

NMSF-SP2Northwestern Mutual Series Fund, Inc. Prospectus

Active/Passive Very Aggressive Portfolio (formerly Active/Passive All Equity Portfolio) – Summary

where proper action for the Portfolio could be adverse to the interest of the Underlying Portfolios or vice versa.

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Asset Allocation Risk – This Portfolio allocates its investments among multiple investment asset classes and among Underlying Portfolios and ETFs, based upon judgments made by the adviser. The Portfolio could miss attractive investment opportunities by underweighting markets or sectors where there are significant returns, and could lose value by overweighting markets where there are significant declines, or may not correctly predict the times to shift assets from one type of investment to another.

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Commodities Risk – The Portfolio’s investments in ETFs whose performance is linked to the price of an underlying commodity or commodity index may be subject to the risks of investing in physical commodities, including risks posed by regulatory, economic and political developments, weather events, natural disasters and market disruption. Commodity prices may have greater volatility than investments in traditional securities.

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Convertible Securities Risk – Convertible securities (which can be bonds, notes, debentures, preferred stock, or other securities which are convertible into or exercisable for common stock), are subject to both the credit and interest rate risks associated with fixed income securities and to the stock market risk associated with equity securities. The value of a convertible security may not increase or decrease as rapidly as the underlying common stock. The Portfolio may be forced to convert a security before it would otherwise choose, which may have an adverse effect on the Portfolio’s ability to achieve its investment objective.

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Credit Risk – The Portfolio could lose money if the issuer or guarantor of a fixed income security held directly or through an Underlying Portfolio or ETF is unwilling or unable to meet its financial obligations. In addition, changes in an issuer’s credit rating or the market’s perception of an issuer’s creditworthiness may also affect the value of the Portfolio’s investment in that issuer. Changes in credit spreads or improvements in an issuer’s credit quality may increase the risk that an issuer calls outstanding securities prior to their maturity.

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Emerging Markets Risk – Investing in emerging market securities increases foreign investing risk, and may subject the Portfolio to more rapid and extreme changes in the value of its holdings compared with investments made in U.S. securities or in foreign, developed countries. Investments in emerging markets may be subject to political, economic, legal, market, and currency risks. Emerging market securities trade in smaller markets which may experience significant price and market volatility, fluctuations in currency values, interest rates and commodity prices, higher transaction costs, and the increased likelihood of the occurrence of trading difficulties, such as delays in executing, clearing and settling Portfolio transactions or in receiving payment of dividends. Special risks associated with investments in emerging market issuers may include a lack of publicly available information, a lack of uniform disclosure, accounting, financial reporting, and recordkeeping standards, and more limited investor protection provisions when compared with developed economies. Emerging market risks also may include unpredictable and changing political, economic and tax policies, the imposition of capital controls and/or foreign investment limitations by a country, nationalization of businesses, and the imposition of sanctions or restrictions in certain investments by other countries, such as the United States.

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Equity Securities Risk – The value of equity securities held through the Underlying Portfolios and ETFs, such as common and preferred stocks, could decline if the financial condition of the companies an Underlying Portfolio or ETF 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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Exchange Traded Funds Risk – Investing in exchange traded funds (ETFs) may expose the Portfolio to greater risk of loss and price fluctuation than investing directly in a comparable portfolio of stocks comprising the index due to lack of liquidity, the additional expenses incurred as a shareholder in another investment company, and tracking error. ETFs are also subject to the risk that their market prices may trade at a premium or discount to their net asset value, which means the Portfolio will overpay for an ETF’s assets if it is trading at a premium and will get less than the value of the ETF’s assets when selling if it is trading at a discount. An active market for an ETF may not be developed or maintained. Trading of an ETF’s shares may be halted by the exchange, in which case the Portfolio would be unable to sell its ETF shares unless and until trading is resumed. To the extent that authorized participants do not place sufficient creation and redemption orders, an ETF's shares may trade at a premium or discount or may face a trading halt or delisting. Investing in ultra-short bond ETFs may pose greater risk than investments in cash equivalents such as money market mutual funds, including risks associated with the credit quality, duration and interest rate sensitivity of the bond instruments held within such ETFs. Ultra-short bond ETFs may be more volatile and involve greater credit and interest rate risk than money market funds and other cash equivalents. In addition, the price of an ultra-short bond ETF may fluctuate, while the goal of a money market mutual fund is to maintain its NAV at a stable $1 per share. Ultra-short bond ETFs may lose money despite a stated investment objective of returning capital.

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Foreign Investing Risk – Exposure to investments in foreign securities, including through Underlying Portfolios and ETFs, 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.

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Fund of Funds Investing Risk – The Portfolio’s investment performance is significantly impacted by the investment performance of the Underlying Portfolios and ETFs it holds. The ability of the Portfolio to meet its investment objective is

NMSF-SP3Northwestern Mutual Series Fund, Inc. Prospectus

Active/Passive Very Aggressive Portfolio (formerly Active/Passive All Equity Portfolio) – Summary

related to the ability of the Underlying Portfolios or ETFs to meet their respective investment objectives as well as the adviser’s allocation decisions with respect to the Underlying Portfolios and ETFs. Each of the Underlying Portfolios and ETFs has its own investment risks, and the Portfolio is indirectly exposed to all the risks of the Underlying Portfolios and ETFs in direct proportion to the amount of assets the Portfolio allocates to each Underlying Portfolio and each ETF. To the extent that the Portfolio invests a significant portion of its assets in a single Underlying Portfolio or ETF, it will be particularly sensitive to the risks associated with that Underlying Portfolio or ETF. Changes in the value of that Underlying Portfolio or ETF may have a significant effect on the Portfolio’s net asset value. The Portfolio will bear a pro rata share of the Underlying Portfolios’ or ETFs’ expenses.

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Geographic Focus Risk – The Portfolio’s performance could be more volatile than that of a more geographically diversified fund and could be significantly impacted as a result of the Portfolio investing a relatively large percentage of its assets in issuers located in a single country, a small number of countries, or a particular geographic region. Also, the Portfolio’s performance may be more closely tied to the market, currency, economic, political, or regulatory conditions in those countries or that region. Similarly, the extent to which an Underlying Portfolio or ETF invests a significant portion of its assets in a single country, a small number of countries or a particular geographic region, may also adversely impact the Portfolio, depending on the Portfolio’s level of investment in that Underlying Portfolio or ETF.

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Investment Style Risk – The Portfolio is subject to risks associated with an Underlying Portfolio’s or ETF's particular style of investing and may underperform with respect to its allocation to the Underlying Portfolio or ETF when the market does not favor that particular investment style. 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 – Exposure to investments 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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Large Transaction Risk – The Underlying Portfolios are used as investments for certain fund of funds, including the Portfolio, and may have a large percentage of their shares owned by such funds. Large redemption activity by the Portfolio or another fund of funds could result in the Underlying Portfolio being forced to sell portfolio securities at a loss to meet redemptions. The adviser may coordinate directly with the portfolio managers of the Underlying Portfolios to attempt to ensure that transactions are accommodated efficiently, including possibly implementing trades over a period of days rather than all at once. These practices may temporarily affect the adviser’s ability to fully implement the Portfolio’s investment strategies.

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Liquidity Risk – Particular investments, such as small stocks, fixed income securities, foreign securities, in particular emerging markets securities, and derivatives to which the Portfolio has exposure, can be difficult to purchase or sell at an advantageous time or price, if at all. These risks may be magnified during periods of economic turmoil or in an extended economic downturn.

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Market Risk – The risk that the market price of securities owned by the Portfolio or an Underlying Portfolio or ETF in which the Portfolio invests 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, 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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Preferred Stocks Risk – Preferred stocks often lack a fixed maturity or redemption date and are therefore more susceptible to price fluctuations when interest rates change. They also carry a greater risk of non-receipt of income because unlike interest on debt securities, dividends on preferred stocks must be declared by the issuer’s board of directors before becoming payable.

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REITs Risk – Investments in REITs are subject to the types of risks associated with investing in the real estate industry, including, among other risks: adverse developments affecting the real estate industry; declines in real property values; changes in interest rates; defaults by mortgagors or other borrowers and tenants; lack of availability of mortgage funds or financing; extended vacancies of properties, especially during economic downturns; casualty or condemnation losses; property taxes and operating losses; and governmental actions, such as changes to tax laws, zoning regulations or environmental regulations. REITs are dependent upon the quality of their management, may have limited financial resources and heavy cash flow dependency, may not be diversified geographically or by property type, and may be subject to self-liquidation.

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Sector Focus Risk – To the extent the Portfolio invests in Underlying Portfolios or ETFs with 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 invests in Underlying Portfolios or ETFs that are underweight in other sectors, the Portfolio risks missing out on advances in those sectors.

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Small and Mid Cap Company Risk – Exposure to investments in small and 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,

NMSF-SP4Northwestern Mutual Series Fund, Inc. Prospectus

Active/Passive Very Aggressive Portfolio (formerly Active/Passive All Equity Portfolio) – Summary

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.

PERFORMANCE

Performance information is not available for the Portfolio because it does not yet have a full calendar year of performance history.

PORTFOLIO MANAGEMENT

Investment Adviser: Mason Street Advisors, LLC (MSA)
Portfolio Managers: James Fleming, CFA, Portfolio Manager of MSA, joined MSA in 2008 and has managed the Portfolio since 2025.
Rodney Schmucker, CFA, Senior Research & Portfolio Analyst of MSA, joined MSA in 2008 and has managed the Portfolio since 2025.
Brent G. Schutte, CFA, Chief Investment Officer of MSA, joined MSA in December 2021 and has managed the Portfolio since 2025.
Garrett D. Aird, CFA, Vice President of MSA, joined MSA in December 2021 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-SP5Northwestern Mutual Series Fund, Inc. Prospectus