Long-Term U.S. Government Bond 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 seek maximum total return, consistent with preservation of capital and prudent investment management.
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
0.55%
Distribution and Service (12b-1) Fees
None
Other Expenses(1)
1.95%
Interest Expense
1.80%
Other Operating Expense
0.15%
Total Annual Portfolio Operating Expenses(1)
2.50%
Expense Reimbursement(2)
(0.05)%
Total Annual Portfolio Operating Expenses After
Expense Reimbursement(1),(2)
2.45%
(1)
“Other Expenses” include interest expense of 1.80%. Interest expense is borne by the Portfolio separately from the management fees paid to the Portfolio’s investment adviser, Mason Street Advisors, LLC, and the Portfolio’s sub-adviser, Pacific Investment Management Company LLC. Excluding interest expense, Total Annual Portfolio Operating Expenses are 0.70% and Total Annual Portfolio Operating Expenses After Expense Reimbursement are 0.65%.
(2)
The Portfolio's investment adviser has contractually agreed to reimburse expenses in order to limit Total Annual Portfolio Operating Expenses After 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 0.65% of the Portfolio's average net assets. 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 expense reimbursement 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
$248
$774
$1,326
$2,832
NMSF-SP1Northwestern Mutual Series Fund, Inc.
Long-Term U.S. Government Bond Portfolio – Summary
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 14% 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 debt securities that are issued or guaranteed by the U.S. Government, its agencies or government sponsored enterprises (“U.S. Government Securities”). Government sponsored enterprises include, for example, the Federal National Mortgage Association (Fannie Mae), the Federal Home Loan Mortgage Corp. (Freddie Mac), the Federal Home Loan Bank (FHLB), the Farm Credit System (FCS) and the Federal Agricultural Mortgage Corp. (Farmer Mac). U.S. Government Securities may be represented by forwards or derivatives such as options, futures contracts or interest rate swap agreements (to take a position on interest rates moving either up or down) that provide exposure to U.S. Government Securities. Assets not invested in U.S. Government Securities may be invested in other types of investment grade fixed income instruments, such as corporate debt securities of U.S. issuers and mortgage- and asset-backed securities, subject to the quality restrictions described below. Mortgage-related securities may include mortgage pass-through securities, collateralized mortgage obligations, commercial mortgage-backed securities and mortgage dollar rolls. The Portfolio may also invest up to 10% of its net assets in preferred stocks.
The Portfolio will normally have a minimum average portfolio duration of eight years and, for point of reference, the dollar-weighted average maturity of the Portfolio, under normal circumstances, is expected to be more than ten years. Duration is a measure of the sensitivity of the price of the Portfolio’s fixed income securities to changes in interest rates; the longer the duration, the more sensitive the price will be to changes in interest rates.
The Portfolio may invest all of its assets in derivative instruments, such as options, futures contracts or interest rate swap agreements (to take a position on interest rates moving either up or down), or in mortgage- or asset-backed securities, subject to the Portfolio’s objective and the Fund’s policies including the 80% investment policy. The adviser may invest in derivatives at any time it deems appropriate. It will generally do so when it believes that U.S. Government Securities are overvalued relative to derivative instruments or to adjust the overall duration of the Portfolio. The potential leverage created by use of derivatives may cause the Portfolio to be more sensitive to interest rate movements and thus more volatile than other long-term U.S. government bond funds that do not use derivatives.
The Portfolio may purchase or sell securities on a when-issued, delayed delivery or forward commitment basis and may engage in short sales. A short sale involves the sale of a security that is borrowed from a broker or other institution, and which must be purchased in the market at a later date and returned to the lender. The Portfolio may invest in repurchase agreements; however, it may not invest more than 10% of its total assets in repurchase agreements which have maturities of more than seven days, nor invest in any repurchase agreements with maturities over 30 days. The Portfolio may, without limitation, seek to obtain market exposure to the securities in which it primarily invests by entering into a series of purchase and sale contracts or by using other investment techniques (such as buy backs or dollar rolls).
The “total return” sought by the Portfolio consists of income earned on the Portfolio’s investments, plus capital appreciation, if any, which generally arises from decreases in interest rates or improving credit fundamentals for a particular sector or security. The Portfolio may engage in frequent and active trading of portfolio securities to achieve its investment objective, particularly during periods of volatile market movements.
The Portfolio’s investments in fixed income securities are limited to investment grade U.S. dollar denominated securities of U.S. issuers that are rated at least A by Moody’s or equivalently rated by S&P or Fitch, or, if unrated, determined by the adviser to be of comparable quality. If a downgrade in the rating of a security in which the Portfolio is invested causes it to fall outside these parameters, the adviser will sell the impacted security as soon as reasonably practicable. In addition, with respect to the Portfolio’s investments in fixed income securities that are not U.S. Government Securities, (and subject to the requirement that the Portfolio under normal circumstances maintains at least 80% of net assets (plus any borrowing for investment purposes) in U.S. Government Securities) the Portfolio may only invest up to 10% of its total assets in securities rated A by Moody’s or equivalently rated by S&P or Fitch, or, if unrated, determined by the adviser to be of comparable quality, and may only invest up to 25% of its total assets in securities rated Aa by Moody’s or equivalently rated by S&P or Fitch or, if unrated, determined by the adviser to be of comparable quality.
The Portfolio may sell a position when, in the adviser’s opinion, it no longer represents a good value, when a superior risk/return opportunity exists in a substitute position, or when it no longer fits within the Portfolio’s macroeconomic or structural strategy.
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.
•
Active Management Risk – The adviser’s investment strategies and techniques may not perform as expected and the adviser’s quality determinations with respect to securities that are unrated by the major credit rating agencies may be inaccurate, which
NMSF-SP2Northwestern Mutual Series Fund, Inc. Prospectus
Long-Term U.S. Government Bond Portfolio – Summary
could cause the Portfolio to underperform other mutual funds or lose money.
•
Counterparty Risk – The Portfolio may sustain a loss in the event the other party(s) in an agreement or a participant to a transaction, such as a broker or swap counterparty, defaults on a contract or fails to perform by failing to pay amounts due, failing to fulfill delivery conditions, or failing to otherwise comply with the terms of the contract. Counterparty risk is inherent in many transactions, including derivatives transactions.
•
Credit Risk – The Portfolio could lose money if the issuer or guarantor of a fixed income security or the counterparty to a derivatives contract 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.
•
Derivatives Risk – The value of a derivative generally depends upon, or is derived from, an underlying asset, reference rate or index. The Portfolio’s use of derivatives involves risks different from, or possibly greater than, the risks associated with investing directly in securities or other traditional investments. Investments in derivatives may not have the intended effects and may result in losses for the Portfolio that may not otherwise have occurred or missed opportunities for the Portfolio. Certain derivatives involve leverage, which could cause the Portfolio to lose more than the principal amount invested. The derivatives could involve management, credit, interest rate, liquidity and market risks, and the risks of misplacing or improper valuation. Changes in the value of the derivative may not correlate as intended with the underlying asset, rate or index. In addition, the Portfolio could sustain a loss in the event the counterparty to a derivatives transaction fails to make the required payments or otherwise comply with the terms of the contract. 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. The Portfolio’s purchase of swap agreements may pose risk arising from losses if the underlying reference asset does not perform as anticipated; such agreements are also subject to counterparty credit, liquidity and leveraging risks. The Portfolio’s use of options involve risk related to the direction and timing of market movements in the price of the underlying asset, obligations related to exercise of the option, and potential loss in value of the initial investment.
•
Equity Securities Risk – The value of equity securities, such as common and preferred 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.
•
High Portfolio Turnover Risk – Active and frequent trading may cause higher brokerage expenses and other transaction costs, which may adversely affect the Portfolio’s performance.
•
Inflation Risk – Your investment in the Portfolio may not provide enough income to keep pace with inflation.
•
Interest Rate Risk – Prices of fixed income instruments generally rise and fall in response to changes in market interest rates. In a rising interest rate environment, the value of the Portfolio’s fixed income investments is likely to decline. A significant rise in interest rates over a short period of time could cause significant losses in the market value of the Portfolio’s fixed income instruments. A Portfolio with a longer average portfolio duration will be more sensitive to changes in interest rates than a portfolio with a shorter average portfolio duration. For example, the market value of a fixed income portfolio with an average duration of five years generally would be expected to fall approximately 5% if interest rates rose by one percentage point. Declining interest rates may increase the risk that an issuer calls outstanding securities prior to their maturity.
•
Issuer Risk – The risk that the value of a security may decline for a reason directly related to the issuer, such as management performance, financial leverage and reduced demand for the issuer’s goods or services.
•
Leverage Risk – Certain transactions, such as when issued, delayed delivery or forward commitments transactions, or the use of derivative transactions, may give rise to leverage, causing more volatility than if the Portfolio had not been leveraged.
•
Liquidity Risk – Fixed income and derivative investments can be difficult to purchase or sell at an advantageous time or price, if at all, during periods of reduced marketability for the investment or due to the size of the transaction. These risks may be magnified during periods of economic turmoil or in an extended economic downturn.
•
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.
NMSF-SP3Northwestern Mutual Series Fund, Inc. Prospectus
Long-Term U.S. Government Bond Portfolio – Summary
•
Mortgage- and Asset-Backed Securities Risk – The risks of investing in mortgage-related and other asset-backed securities, including interest rate risk, credit risk, liquidity risk, prepayment risk and extension risk. Mortgage-related and other asset-backed securities represent interests in pools of mortgages or other assets and often involve risks that are different or possibly more acute than risks associated with other types of debt instruments. The value of some mortgage- or asset-backed securities may be particularly sensitive to changes in prevailing interest rates. The use of mortgage dollar rolls involves potential risks of loss that are different from those related to the mortgage securities underlying the transactions, including counterparty risk, market risk, and financial risk (including the risk that the value of the principal and interest payments associated with the mortgage instrument sold to a counterparty exceeds the compensation paid to the Portfolio by the counterparty).
•
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.
•
Prepayment and Extension Risk – Prepayment risk is the risk that principal on a debt obligation will be paid earlier than scheduled or expected, which could reduce yield and market value of the security and shorten the Portfolio’s average effective maturity. The rate of prepayments tends to increase as interest rates fall. Extension risk is the risk that, as interest rates rise, repayments on a debt obligation may occur more slowly than anticipated by the market and the obligation may remain outstanding longer.
•
Repurchase Agreements Risk – If the other party to a repurchase agreement defaults on its obligation under the agreement, the Portfolio may suffer delays and incur costs or lose money in exercising its rights under the agreement. These risks may be heightened if the other party is located outside of the U.S. If the seller fails to repurchase the security and the market value of the security declines, the Portfolio may lose money.
•
Short Sale Risk – The risk of entering into short sales, including the potential loss of more money than the actual cost of the investment, and the risk that the third party to the short sale may fail to honor its contract terms, causing a loss to the Portfolio.
•
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.
•
U.S. Government Securities Risk – Not all obligations of the U.S. government, its agencies and instrumentalities are backed by the full faith and credit of the U.S. Treasury. Some obligations are backed only by the credit of the issuing agency or instrumentality, and in some cases there may be some risk of default by the issuer. Any guarantee by the U.S. government or its agencies or instrumentalities of a security held by the Portfolio does not apply to the market value of such security or to shares of the Portfolio itself.
•
When Issued, Delayed Delivery and Forward Commitment Risk – When issued, delayed delivery purchases and forward commitment transactions involve a risk of loss if the value of the securities declines prior to the settlement date. This risk is in addition to the risk that the Portfolio’s other assets will decline in value. Therefore, these transactions may result in a form of leverage and increase the Portfolio’s overall investment expense.
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. 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.
NMSF-SP4Northwestern Mutual Series Fund, Inc. Prospectus
Long-Term U.S. Government Bond Portfolio – Summary
  
 
Quarter/Year
Return
Best Quarter
1st quarter, 2020
20.64%
Worst Quarter
1st quarter, 2021
-13.26%
Average Annual Total Return
(for periods ended December 31, 2025) 
 
 
1 Yr
5 Yr
10 Yr
Portfolio:
Long-Term U.S. Government
Bond Portfolio
6.15%
-7.18%
-0.19%
Strategy Index:
Bloomberg® Long-Term Treasury
Index
5.59%
-7.23%
0.02%
Broad-Based
Index:
Bloomberg® U.S. Aggregate
Bond Index
(reflects no deduction for fees,
expenses or taxes)
7.30%
-0.36%
2.01%
PORTFOLIO MANAGEMENT
Investment Adviser: Mason Street Advisors, LLC
Sub-Adviser: Pacific Investment Management Company LLC (PIMCO)
Portfolio Managers: Michael Cudzil joined PIMCO in 2012 and is a Managing Director in PIMCO’s Newport Beach office. He has managed the Portfolio since 2016.
Tim Crowley joined PIMCO in 2008 and is an Executive Vice President in PIMCO’s Newport Beach office. He 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