Index 600 Stock 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 to achieve investment results that approximate the performance of the Standard & Poor’s SmallCap 600® Index (“S&P SmallCap 600® Index”).

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	0.22%
Distribution and Service (12b-1) Fees	None
Other Expenses	0.04%
Total Annual Portfolio Operating Expenses	0.26%

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. 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
$27	$84	$146	$331

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 24% of the average value of its portfolio.

PRINCIPAL INVESTMENT STRATEGIES

The Portfolio employs a “passive management,” or indexing, investment approach designed to track the performance of the S&P SmallCap 600® Index. S&P SmallCap 600® Index is composed of domestic stocks with market capitalizations ranging between approximately $442.41 million and $8.78 billion as of March 31, 2026. Normally, the Portfolio invests at least 80% of net assets (plus any borrowings for investment purposes) in stocks included in the underlying index. Further, the Portfolio attempts to achieve its objective by investing all, or substantially all, of its assets in stocks that make up the S&P SmallCap 600® Index, holding each stock in approximately the same proportion as its weighting in the Index. This is known as a full replication strategy. The Portfolio may also invest in equity index futures and exchange traded funds for cash management and liquidity purposes and to help achieve full replication.

Standard & Poor’s constructs the index by first identifying major industry categories and then allocating a representative sample of the larger and more liquid stocks in those industries to the index. S&P weights each stock according to its float-adjusted market value. For example, the 50 largest companies in the index may account for over 50% of its value.

The approach employed by the Portfolio with respect to reconstitution and rebalancing aligns with the process followed generally by the S&P SmallCap 600® Index. Changes to the underlying company constituents of the S&P SmallCap 600® Index are made on an as-needed basis and are usually announced several days before they are scheduled to be implemented. The S&P SmallCap 600® Index typically makes weightings adjustments based on changes in the amount of a constituent company’s shares outstanding on a quarterly basis. The constituent and share-based weightings changes made by S&P SmallCap 600® Index will be made in a parallel fashion by the Portfolio on substantially the same timeline.

NMSF-SP1Northwestern Mutual Series Fund, Inc.

Index 600 Stock Portfolio – Summary

Because the Portfolio pursues an investment strategy of full replication of the underlying S&P SmallCap 600® Index, investments in the Portfolio will be concentrated in a particular industry only to the extent that the S&P SmallCap 600® Index is also concentrated in that industry. Similarly, to the extent that the underlying S&P SmallCap 600® Index has a relatively high percentage of its assets invested in a particular sector, the Portfolio may also hold a relatively high percentage of its assets in that particular sector, as well.

The Portfolio utilizes cash equitization instruments, and rebalancing occurs as necessary to maintain balances within established target ranges for these instruments. Cash equitization as employed by the Portfolio is a strategy pursuant to which the Portfolio will acquire certain derivative instruments, such as equity index futures, whereby the futures will be utilized to replicate index returns and enable the Portfolio to be fully invested, while still maintaining cash balances as needed by the Portfolio. This strategy works to limit the extent to which uninvested cash will weigh negatively on overall Portfolio returns when markets go up.

The Index 600 Stock Portfolio’s ability to match the performance of the S&P SmallCap 600® Index will be affected to some extent by the size and timing of cash flows into and out of the Index 600 Stock Portfolio. The Portfolio will be managed with a view to reducing such effects.

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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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, and the risk of adverse price movements in the market. Certain derivatives involve leverage, which could cause the Portfolio to lose more than the principal amount invested. Other risks include counterparty and liquidity risks. The Portfolio’s purchase of 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.

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Equity Securities Risk – The value of equity securities, such as the stocks in which the Portfolio invests, 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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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.

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Indexing Strategy Risk – A Portfolio using a passive management strategy is not “actively” managed, and therefore does not engage in shifting portfolio assets to take advantage of market opportunity, and does not attempt to manage market volatility, use defensive strategies or reduce the effects of any long-term periods of poor stock performance. Since the Portfolio implements a full replication strategy with respect to the index which it tracks, to the extent the index has a significant allocation to a particular sector, industry, or group of industries, or to a single issuer or small number of issuers, the Portfolio will exhibit a significant investment level in that industry, group of industries, sector, issuer or small number of issuers. Portfolio performance may be adversely affected by a significant investment in a sector, industry, or group of industries, or an issuer or small number of issuers, and may be more susceptible to adverse economic, market, political or regulatory developments affecting the sector, industry, group of industries, or issuer(s) subject to a significant level of investment. Information about the Portfolio’s exposure to a particular sector, industry, or group of industries (as applicable) is available in the Portfolio’s Annual and Semi-Annual Reports to Shareholders and on required forms filed with the SEC.

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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, 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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Small Cap Company Risk – Investing in small 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

NMSF-SP2Northwestern Mutual Series Fund, Inc. Prospectus

Index 600 Stock Portfolio – Summary

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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Tracking Error Risk – The Portfolio may be subject to tracking error, which is the divergence of the Portfolio’s performance from that of the underlying index. Tracking error may occur due to a number of factors, including differences between the securities held in the Portfolio and those included in the underlying index, and based upon the fact that the Portfolio incurs fees and expenses, while the underlying index does not. Tracking error risk may be heightened during times of increased market volatility or other unusual market conditions.

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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.

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 February 1, 2021, the Portfolio was managed by Mason Street Advisors, LLC. 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	4th quarter, 2020	31.17%
Worst Quarter	1st quarter, 2020	-32.66%

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

		1 Yr	5 Yr	10 Yr
Portfolio:	Index 600 Stock Portfolio	5.78%	6.98%	9.48%
Strategy Index:	S&P SmallCap 600® Index (reflects no deduction for fees, expenses or taxes)	6.02%	7.31%	9.81%
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: Northern Trust Investments, Inc. (Northern Trust)
Portfolio Managers: Keith Carroll, Portfolio Manager, joined Northern Trust in 2007 and has managed the Portfolio since 2021.
Lucy Johnston, Portfolio Manager, joined Northern Trust in 1997 and has managed the Portfolio since 2024.

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-SP3Northwestern Mutual Series Fund, Inc. Prospectus