STATE STREET INSTITUTIONAL INVESTMENT TRUST

State Street Institutional Liquid Reserves Fund

Bancroft Capital Class (VTDXX)

Administration Class (SSYXX)

Institutional Class (SSHXX)

Investment Class (SSVXX)

Investor Class (SSZXX)

Opportunity Class (OPIXX)

Premier Class (SSIXX)

(the “Fund”)

Supplement dated January 24, 2025 to the

Summary Prospectuses, Prospectuses and Statements of Additional Information (the “SAIs”), each dated April 30, 2024, as may be supplemented and/or revised from time to time

The Board of Trustees (“Board”) has approved the conversion of the Fund to qualify as a “government money market fund” as defined in Rule 2a-7 under the Investment Company Act of 1940, as amended. As a result, the Fund will be renamed the State Street Institutional Liquid Reserves Government Money Market Fund and will seek to achieve its investment objective by investing substantially all of its investable assets in the Street U.S. Government Money Market Portfolio (the “U.S. Government Portfolio” or sometimes referred to in context as the “Portfolio”). As a government money market fund, the Fund has not elected to be subject to the liquidity fee provision of Rule 2a‑7 and it will seek to maintain a $1.00 per share net asset value using the amortized cost valuation method, in compliance with the risk limiting conditions of Rule 2a‑7, to value its portfolio instruments. In connection with its transition from using a per share net asset value calculated to four decimal places to one calculated to two decimal places, on the Effective Date the Fund will execute a share split with the goal of establishing a $1.00 per share net asset value.

The Fund expects that these changes will become effective on or about March 7, 2025 (the “Effective Date”). As of the Effective Date, all references to the Fund will be updated to reflect its operations as a government money market fund.

On the Effective Date, the Fund’s Summary Prospectuses, Prospectuses and SAIs are revised as follows:

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All references to the State Street Money Market Portfolio will be replaced with the State Street U.S. Government Money Market Portfolio.

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All references to the State Street Institutional Liquid Reserves Fund will be replaced with the State Street Institutional Liquid Reserves Government Money Market Fund.

Summary Prospectuses and Prospectuses

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The section titled “Principal Investment Strategies” in the Summary Prospectuses and the sub‑section titled “Principal Investment Strategies” within the “Fund Summaries – State Street Institutional Liquid Reserves Fund” section of the Prospectuses are hereby deleted and replaced with the following:

The State Street Institutional Liquid Reserves Government Money Market Fund (the “ILR Government Fund”) is a government money market fund and invests only in obligations issued or guaranteed as to principal and/or interest, as applicable, by the U.S. government or its agencies and instrumentalities, as well as repurchase agreements secured by such instruments. The Fund may hold a portion of its assets in cash pending investment, to satisfy redemption requests or to meet the Fund’s other cash management needs.

The Fund follows a disciplined investment process that attempts to provide stability of principal, liquidity and current income, by investing in U.S. government securities. Among other things, SSGA Funds Management, Inc. (“SSGA FM” or the “Adviser”), the investment adviser to the Fund, conducts its own credit analyses of potential investments and portfolio holdings, and relies substantially on a dedicated short-term credit research team. The Fund invests in accordance with regulatory requirements applicable to money market funds. Regulations require, among other things, a money market fund to invest only in short-term, high quality debt obligations (generally, securities that have remaining maturities of 397 calendar days or less, with the exception of certain floating rate securities that may have final maturities longer than 397 days but use maturity shortening provisions to meet the 397 day requirement, and that the Fund believes present minimal credit risk), to maintain a maximum dollar-weighted average maturity and dollar-weighted average life of sixty (60) days or less and 120 days or less, respectively, and to meet requirements as to portfolio diversification and liquidity. All securities held by the Fund are U.S. dollar-denominated, and they may have fixed, variable or floating interest rates.

The Fund attempts to meet its investment objective by investing in:

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Obligations issued or guaranteed as to principal and/or interest, as applicable, by the U.S. government or its agencies and instrumentalities, such as U.S. Treasury securities and securities issued by the Government National Mortgage Association (“GNMA”), which are backed by the full faith and credit of the United States;

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Obligations issued or guaranteed by the Federal Home Loan Mortgage Corporation, the Federal National Mortgage Association, and U.S. government-sponsored entities such as the Federal Home Loan Bank, and the Federal Farm Credit Banks Funding Corporation, which are not backed by the full faith and credit of the United States; and

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Repurchase agreements collateralized by U.S. government securities.

The Fund seeks to achieve its investment objective by investing substantially all of its investable assets in the U.S. Government Portfolio, which has substantially identical investment policies to the Fund. When the Fund invests in this “master-feeder” structure, the Fund’s only investments are shares of the Portfolio, and it participates in the investment returns achieved by the Portfolio. Descriptions in this section of the investment activities of the “Fund” also generally describe the expected investment activities of the Portfolio.

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The section titled “Principal Risks” in the Summary Prospectuses and the sub‑section titled “Principal Risks” within the “Fund Summaries – State Street Institutional Liquid Reserves Fund” section of the Prospectuses are hereby deleted and replaced with the following:

The Fund is subject to the following risks. You could lose money by investing in the Fund. Although the Fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. An investment in the Fund is subject to investment risks, including possible loss of principal, is not a deposit in a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation (“FDIC”) or any other government agency. The Fund’s sponsor is not required to reimburse the Fund for losses, and you should not expect that the sponsor will provide financial support to the Fund at any time, including during periods of market stress. Certain risks relating to instruments and strategies used in the management of the Fund are placed first. The significance of any specific risk to an investment in the Fund will vary over time, depending on the composition of the Fund’s portfolio, market conditions, and other factors. You should read all of the risk information presented below carefully, because any one or more of these risks may result in losses to the Fund.

Money Market Risk: An investment in a money market fund is not a deposit of any bank and is not insured or guaranteed by the FDIC or any other government agency. Although a money market fund generally seeks to preserve the value of its shares at $1.00 per share, there can be no assurance that it will do so, and it is possible to lose money by investing in a money market fund. A major or unexpected change in interest rates or a decline in the credit quality of an issuer or entity providing credit support, an inactive trading market for money market instruments, or adverse market, economic, industry, political, regulatory, geopolitical, and other conditions could cause a money market fund’s share price to fall below $1.00.

U.S. Government Securities Risk: Certain U.S. government securities are supported by the full faith and credit of the United States; others are supported by the right of the issuer to borrow from the U.S. Treasury; others are supported by the discretionary authority of the U.S. government to purchase the agency’s obligations; and still others are supported only by the credit of the issuing agency, instrumentality, or enterprise. Although U.S. government-sponsored enterprises such as the Federal Home Loan Mortgage Corporation (“Freddie Mac”) and the Federal National Mortgage Association (“Fannie Mae”) may be chartered or sponsored by Congress, they are not funded by Congressional appropriations, and their securities are not issued by the U.S. Treasury, are not supported by the full faith and credit of the U.S. government, and involve increased credit risks.

Repurchase Agreement Risk: Repurchase agreements may be viewed as loans made by the Fund which are collateralized by the securities subject to repurchase. If the Fund’s counterparty should default on its obligations and the Fund is delayed or prevented from recovering the collateral, or if the value of the collateral is insufficient, the Fund may realize a loss.

Stable Share Price Risk: If the market value of one or more of the Fund’s investments changes substantially, the Fund may not be able to maintain a stable share price of $1.00. This risk typically is higher during periods of rapidly changing interest rates or when issuer credit quality generally is falling, and is made worse when the Fund experiences significant redemption requests.

Market Risk: The Fund’s investments are subject to changes in general economic conditions, general market fluctuations and the risks inherent in investment in securities markets. Investment markets can be volatile and prices of investments can change substantially due to various factors including, but not limited to, economic growth or recession, changes in interest rates, inflation, changes in the actual or perceived creditworthiness of issuers, and general market liquidity. The Fund is subject to the risk that geopolitical events will disrupt securities markets and adversely affect global economies and markets. Local, regional or global events such as war, military conflicts, acts of terrorism, natural disasters, the spread of infectious illness or other public health issues, or other events could have a significant impact on the Fund and its investments.

Low Short-Term Interest Rates: During market conditions in which short-term interest rates are at low levels, the Fund’s yield can be very low. During these conditions, it is possible that the Fund will generate an insufficient amount of income to pay its expenses, and that it will not be able to pay a daily dividend and may have a negative yield (i.e., it may lose money on an operating basis). It is possible that the Fund would, during these conditions, maintain a substantial portion of its assets in cash, on which it may earn little, if any, income.

Counterparty Risk: The Fund will be subject to credit risk with respect to the counterparties with which the Fund enters into repurchase agreements and other transactions. If a counterparty fails to meet its contractual obligations, the Fund may be unable to terminate the transaction, and it may be delayed or prevented from realizing on any collateral in the event of a bankruptcy or insolvency proceeding relating to the counterparty.

Debt Securities Risk: The values of debt securities may increase or decrease as a result of the following: market fluctuations, changes in interest rates, actual or perceived inability or unwillingness of issuers, guarantors or liquidity providers to make scheduled principal or interest payments, or illiquidity in debt securities markets. To the extent that interest rates rise, certain underlying obligations may be paid off substantially slower than originally anticipated and the value of those securities may fall sharply. A rising interest rate environment may cause the value of the Fund’s fixed income securities to decrease, an adverse impact on the liquidity of the Fund’s fixed income securities, and increased volatility of the fixed income markets. During periods when interest rates are at low levels, the Fund’s yield can be low, and the Fund may have a negative yield (i.e., it may lose money on an operating basis). To the extent that interest rates fall, certain underlying obligations may be paid off substantially faster than originally anticipated. If the principal on a debt obligation is prepaid before expected, the prepayments of principal may have to be reinvested in obligations paying interest at lower rates. During periods of falling interest rates, the income received by the Fund may decline. Changes in interest rates will likely have a greater effect on the values of debt securities of longer durations. Returns on investments in debt securities could trail the returns on other investment options, including investments in equity securities. High levels of inflation and/or a significantly changing interest rate environment can lead to heightened levels of volatility and reduced liquidity.

Interest Rate Risk: Interest rate risk is the risk that debt securities will decline in value because of increases in interest rates. The value of a security with a longer duration will be more sensitive to changes in interest rates than a similar security with a shorter duration. Interest-only and principal-only securities are especially sensitive to interest rate changes, which can affect not only their prices but can also change the income flows and repayment assumptions about those investments.

Income Risk: The Fund’s income may decline due to falling interest rates or other factors. Issuers of securities held by the Fund may call or redeem the securities during periods of falling interest rates, and the Fund would likely be required to reinvest in securities paying lower interest rates. If an obligation held by the Fund is prepaid, the Fund may have to reinvest the prepayment in other obligations paying income at lower rates.

Large Shareholder Risk: To the extent a large proportion of the interests of the Portfolio are held by a small number of investors (or a single investor), including funds or accounts over which the Adviser has investment discretion, the Portfolio is subject to the risk that these investors will purchase or redeem Portfolio interests in large amounts rapidly or unexpectedly, including as a result of an asset allocation decision made by the Adviser. These transactions could adversely affect the ability of the Portfolio to conduct its investment program.

Master/Feeder Structure Risk: The Fund pursues its objective by investing substantially all of its assets in another pooled investment vehicle (a “master fund”). The ability of the Fund to meet its investment objective is directly related to the ability of the master fund to meet its investment objective. The Adviser serves as investment adviser to the master fund, leading to potential conflicts of interest. The Fund will bear its pro rata portion of the expenses incurred by the master fund. Substantial redemptions by other investors in a master fund may affect the master fund’s investment program adversely and limit the ability of the master fund to achieve its objective.

Mortgage-Related and Other Asset-Backed Securities Risk: Investments in mortgage-related and other asset-backed securities are subject to the risk of significant credit downgrades, illiquidity, and defaults to a greater extent than many other types of fixed-income investments. The liquidity of mortgage-related and asset-backed securities may change over time. During periods of falling interest rates, mortgage- and asset-backed securities may be called or prepaid,

which may result in the Fund having to reinvest proceeds in other investments at a lower interest rate. During periods of rising interest rates, the average life of mortgage- and asset-backed securities may extend, which may lock in a below-market interest rate, increase the security’s duration and interest rate sensitivity, and reduce the value of the security. Enforcing rights against the underlying assets or collateral may be difficult, and the underlying assets or collateral may be insufficient if the issuer defaults.

Rapid Changes in Interest Rates Risk: Rapid changes in interest rates may cause significant requests to redeem Fund Shares, and possibly cause the Fund to sell portfolio securities at a loss to satisfy those requests.

Significant Exposure to U.S. Government Agencies or Instrumentalities Risk: To the extent the Fund focuses its investments in securities issued or guaranteed by U.S. government agencies or instrumentalities, any market movements, regulatory changes or changes in political or economic conditions that affect the U.S. government agencies or instrumentalities in which the Fund invests may have a significant impact on the Fund’s performance. Events that would adversely affect the market prices of securities issued or guaranteed by one government agency or instrumentality may adversely affect the market price of securities issued or guaranteed by other government agencies or instrumentalities.

U.S. Treasury Obligations Risk: U.S. Treasury obligations may differ from other fixed income securities in their interest rates, maturities, times of issuance and other characteristics. Similar to other issuers, changes to the financial condition or credit rating of the U.S. government may cause the value of the Fund’s U.S. Treasury obligations to decline.

Variable and Floating Rate Securities Risk: During periods of increasing interest rates, changes in the coupon rates of variable or floating rate securities may lag behind the changes in market rates or may have limits on the maximum increases in coupon rates. Alternatively, during periods of declining interest rates, the coupon rates on such securities will typically readjust downward resulting in a lower yield.

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The sub‑section titled “ILR Fund – Principal Investment Strategies” within the “Additional Information About Investment Objectives, Principal Strategies And Risks” section of the Prospectuses is hereby deleted and replaced with the following:

The ILR Government Fund is a government money market fund and invests only in obligations issued or guaranteed as to principal and/or interest, as applicable, by the U.S. government or its agencies and instrumentalities, as well as repurchase agreements secured by such instruments. The Fund will provide shareholders with at least sixty (60) days’ prior notice of any change in this investment policy. The Fund may hold a portion of its assets in cash pending investment, to satisfy redemption requests or to meet the Fund’s other cash management needs.

The Fund follows a disciplined investment process that attempts to provide stability of principal, liquidity and current income, by investing in U.S. government securities. Among other things, SSGA FM conducts its own credit analyses of potential investments and portfolio holdings, and relies substantially on a dedicated short-term credit research team. The Fund invests in accordance with regulatory requirements applicable to money market funds. Regulations require, among other things, a money market fund to invest only in short-term, high quality debt obligations (generally, securities that have remaining maturities of 397 calendar days or less, with the exception of certain floating rate securities that may have final maturities longer than 397 days but use maturity shortening provisions to meet the 397 day requirement, and that the Fund believes present minimal credit risk), to maintain a maximum dollar-weighted average maturity and dollar-weighted average life of sixty (60) days or less and 120 days or less, respectively, and to meet requirements as to portfolio diversification and liquidity. All securities held by the Fund are U.S. dollar-denominated, and they may have fixed, variable or floating interest rates.

The Fund attempts to meet its investment objective by investing in:

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Repurchase agreements collateralized by U.S. government securities.

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The sub‑section titled “Additional Information About Risks” within the “Additional Information About Investment Objectives, Principal Strategies And Risks” section of the Prospectuses is hereby revised to remove the following risks as principal risks of the Fund:

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Banking and Companies Risk

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Call/Prepayment Risk

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Extension Risk

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Financial Institutions Risk

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Liquidity Risk

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Money Market Risk-Floating NAV

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Non‑U.S. Securities Risk

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Restricted Securities Risk

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Risk of Investment in Other Pools

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Risks Related to ECDs, ETDs and YCDs

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Section 4(a)(2) Commercial Paper and Rule 144A Securities Risk

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Zero-Coupon Bond Risk

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The sub‑section titled “Additional Information About Risks” within the “Additional Information About Investment Objectives, Principal Strategies And Risks” section of the Prospectuses is hereby revised to add the following risks as principal risks of the Fund:

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Money Market Risk. An investment in a money market fund is not a deposit of any bank and is not insured or guaranteed by the FDIC or any other government agency. Although a money market fund generally seeks to preserve the value of its shares at $1.00 per share, there can be no assurance that it will do so, and it is possible to lose money by investing in a money market fund. A major or unexpected change in interest rates or a decline in the credit quality of an issuer or entity providing credit support, an inactive trading market for money market instruments, or adverse market, economic, industry, political, regulatory, geopolitical, and other conditions could cause a money market fund’s share price to fall below $1.00. It is possible that a money market fund will issue and redeem shares at $1.00 per share at times when the fair value of the money market fund’s portfolio per share is more or less than $1.00. Rule 2a‑7 as adopted by the SEC permits a money market fund to impose a liquidity fee upon the sale of fund shares under certain circumstances, however the Funds are all government money market funds that have not elected to be subject to the liquidity fee provision of Rule 2a‑7. None of State Street Corporation, State Street, State Street Global Advisors, SSGA FM or their affiliates (collectively, the “State Street Entities”) guarantee the value of an investment in a money market fund at $1.00 per share. Investors should have no expectation of capital support to a money market fund from the State Street Entities.

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Significant Exposure to U.S. Government Agencies or Instrumentalities Risk. To the extent a Fund focuses its investments in securities issued or guaranteed by U.S. government agencies or instrumentalities, any market movements, regulatory changes or changes in political or economic conditions that affect the U.S. government agencies or instrumentalities in which the Fund invests may have a significant impact on a Fund’s performance. Events that would adversely affect the market prices of securities issued or guaranteed by one government agency or instrumentality may adversely affect the market price of securities issued or guaranteed by other government agencies or instrumentalities.

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Stable Share Price Risk. If the market value of one or more of the Fund’s investments changes substantially, the Fund may not be able to maintain a stable share price of $1.00. This risk typically is higher during periods of rapidly changing interest rates or when issuer credit quality generally is falling, and is made worse when the Fund experiences significant redemption requests.

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U.S. Treasury Obligations Risk. U.S. Treasury obligations may differ from other securities in their interest rates, maturities, times of issuance and other characteristics. Similar to other issuers, changes to the financial condition or credit rating of the U.S. government may cause the value of a Fund’s U.S. Treasury obligations to decline. The total public debt of the United States as a percent of gross domestic product grew rapidly after the financial crisis of 2008 and has remained at a historically high level. Although high debt levels do not necessarily indicate or cause economic problems, they may create certain systemic risks if sound debt management practices are not implemented. A high national debt level may increase market pressures to meet government funding needs, which may drive debt cost higher and cause a country to sell additional debt, thereby increasing refinancing risk. A high national debt also raises concerns that a government will not be able to make principal or interest payments when they are due. In the worst case, unsustainable debt levels can cause a decline in the value of the dollar (which may lead to inflation), and can prevent the U.S. government from implementing effective countercyclical fiscal policy in economic downturns. On August 5, 2011, Standard & Poor’s Ratings Services downgraded U.S. Treasury securities from AAA rating to AA+ rating. Standard & Poor’s Ratings Services stated that its decision was prompted by its view on the rising public debt burden and its perception of greater policymaking uncertainty. Fitch downgraded its rating of U.S. Treasury securities in August 2023. A downgrade of the ratings of U.S. government debt obligations, which are often used as a benchmark for other borrowing arrangements, could result

in higher interest rates for individual and corporate borrowers, cause disruptions in the international bond markets and have a substantial negative effect on the U.S. economy. Any additional downgrades of U.S. Treasury securities from ratings agencies may cause the value of the Fund’s U.S. Treasury obligations to decline. In recent years, impasses in Congress regarding the federal budget have caused repeated temporary Federal government shutdowns. While Congress has temporarily suspended the debt limit from time to time, the risks that the U.S. government will not adopt a long-term budget or deficit reduction plan, of one or more additional Federal government shutdowns or of future failures to not increase the Federal government’s debt limit, may affect investor and consumer confidence and may adversely impact financial markets and the broader economy, perhaps suddenly and to a significant degree.

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The following risks in the sub‑section titled “Additional Information About Risks” within the “Additional Information About Investment Objectives, Principal Strategies And Risks” section of the Prospectuses are hereby deleted and replaced with the following:

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Money Market Fund Regulatory Risk. Money market funds and the securities they invest in are subject to comprehensive regulations. The SEC has adopted amendments to money market fund regulation that, among other things, increase the daily and weekly liquid asset requirements (“Money Market Fund Reform”). Money Market Fund Reform permits government money market funds (such as the Funds), that are experiencing a negative gross yield as a result of negative interest rates, to either convert from a stable share price to a floating share price or reduce the number of shares outstanding (e.g., through a reverse stock split) to maintain a stable net asset value per share, subject to certain Board determinations and disclosures to investors. The SEC and other government agencies continue to review the regulation of money market funds and may implement additional regulatory changes in the future. The enactment of any new legislation or regulations impacting the money market fund industry could limit the Fund’s investment flexibility and reduce its ability to generate returns.

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Rapid Changes in Interest Rates. The values of instruments held by a Fund may be adversely affected by rapid changes in interest rates. Rapid changes in interest rates may cause significant requests to redeem Fund Shares, and possibly cause a Fund to sell portfolio securities at a loss to satisfy those requests. Significant losses could impair a Fund’s ability to maintain a stable share price of $1.00.

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The section of the Prospectuses titled “Additional Information About Non‑Principal Investment Strategies and Risks” is hereby revised to add the following as a risk of the Fund:

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Risk of Investment in Other Pools. If a Fund invests in another pooled investment vehicle, it is exposed to the risk that the other pool will not perform as expected. A Fund is exposed indirectly to all of the risks applicable to an investment in such other pool. In addition, lack of liquidity in the underlying pool could result in its value being more volatile than the underlying portfolio of securities, and may limit the ability of a Fund to sell or redeem its interest in the pool at a time or at a price it might consider desirable. The investment policies and limitations of the other pool may not be the same as those of the Fund; as a result, the Fund may be subject to additional or different risks, or may achieve a reduced investment return, as a result of its investment in another pool. If a pool is an exchange-traded fund or other product traded on a securities exchange or otherwise actively traded, its shares may trade at a premium or discount to their NAV, an effect that might be more pronounced in less liquid markets. A Fund bears its proportionate share of the fees and expenses of any pool in which it invests. The Adviser or an affiliate may serve as investment adviser to a pool in which the Fund may invest, leading to potential conflicts of interest. For example, the Adviser or its affiliates may receive fees based on the amount of assets invested in the pool. Investment by a Fund in the pool may be beneficial to the Adviser or an affiliate in the management of the pool, by helping to achieve economies of scale or enhancing

cash flows. Due to this and other factors, the Adviser may have an incentive to invest a Fund’s assets in a pool sponsored or managed by the Adviser or its affiliates in lieu of investments by the Fund directly in portfolio securities, or may have an incentive to invest in the pool over a pool sponsored or managed by others. Similarly, the Adviser may have an incentive to delay or decide against the sale of interests held by a Fund in a pool sponsored or managed by the Adviser or its affiliates. It is possible that other clients of the Adviser or its affiliates will purchase or sell interests in a pool sponsored or managed by the Adviser or its affiliates at prices and at times more favorable than those at which a Fund does so.

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The sub‑section titled “For the ILR Fund” in the section of the Prospectuses titled “Shareholder Information – Determination of Net Asset Value” is hereby deleted.

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The sub‑section titled “For the U.S. Government Fund, Treasury Plus Fund and Treasury Fund” in the section of the Prospectuses titled “Shareholder Information – Determination of Net Asset Value” is hereby deleted and replaced with the following:

For all Funds:

The Treasury Plus Fund, the U.S. Government Fund and the ILR Government Fund each determines its NAV per share once each Business Day at 5:00 p.m. ET except for days when the NYSE closes earlier than its regular closing time, in which event those Funds will determines their NAVs at the earlier closing time. The Treasury Fund determines its NAV per share once each Business Day at 2:30 p.m. ET, or the close of the NYSE, whichever is earlier (the time when a Fund determines its NAV per share is referred to herein as the “Valuation Time”). Each Fund seeks to maintain a $1.00 per share NAV and, accordingly, uses the amortized cost valuation method, in compliance with the risk limiting conditions of Rule 2a‑7 under the Investment Company Act of 1940, as amended (the “1940 Act”), to value its portfolio instruments. The amortized cost valuation method initially prices an instrument at its cost and thereafter assumes a constant amortization to maturity of any discount or premium, regardless of the impact of fluctuating interest rates on the market value of the instrument.

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The fifth paragraph of the sub‑section titled “Purchasing Shares” in the section of the Prospectuses titled “Shareholder Information” is hereby deleted and replaced with the following:

Purchase requests received by a Fund in good order (a purchase request is in good order if it meets the requirements implemented from time to time by the Transfer Agent or authorized agent of the Fund, and for new accounts includes submission of a completed and signed application and all documentation necessary to open an account) on a Business Day will, if payment is received by FedWire, be priced at the NAV next determined after the order is accepted by the Fund. Payments received by FedWire prior to the last Valuation Times will earn dividend accrual for that purchase.

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The last paragraph of the sub‑section titled “Purchasing Shares” in the section of the Prospectuses titled “Shareholder Information” is hereby deleted and replaced with the following:

The Funds reserve the right to cease accepting investments at any time or to reject any investment order. In addition, the U.S. Government Fund, the Treasury Plus Fund and the ILR Government Fund may limit the amount of a purchase order received after 3:00 p.m. ET. The Treasury Fund may limit the amount of a purchase order received after 12:00 p.m. (noon) ET.

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The sub‑section titled “Redeeming Shares” in the section of the Prospectuses titled “Shareholder Information” is hereby deleted and replaced with the following:

Redeeming Shares

An investor may redeem all or any portion of its investment. Redemption orders are processed at the NAV next determined after a Fund receives a redemption order in good form.

If the Treasury Fund, the Treasury Plus Fund, the U.S. Government Fund or the ILR Government Fund receives a redemption order in good form prior to 2:30 p.m. ET for the Treasury Fund and 5:00 p.m. ET for the Treasury Plus Fund, U.S. Government Fund and ILR Government Fund on a Business Day, the Fund typically expects to pay out redemption proceeds on that day, but no later than the next Business Day if redemption proceeds are sent by wire or ACH.

If redemption proceeds are sent by check, the Fund pays out redemption proceeds on the next Business Day. If a full redemption order is requested, no dividends will accrue with respect to shares on the day the redemption proceeds are sent. If a redemption order is placed after 2:30 p.m. ET for the Treasury Fund and 5:00 p.m. ET for the U.S. Government Fund, the Treasury Plus Fund and the ILR Government Fund, the Fund typically expects to pay out redemption proceeds on the next Business Day (and dividends will accrue up to, but not including, the day that redemption proceeds are sent), but no later than the following Business Day. The Fund may postpone and/or suspend redemption and payment beyond the foregoing time periods only as follows:

For any period during which there is a non‑routine closure of the Fedwire or applicable Federal Reserve Banks;

For any period (1) during which the NYSE is closed other than customary week‑end and holiday closings or (2) during which trading on the NYSE is restricted;

For any period during which an emergency exists as a result of which (1) disposal of securities owned by the Fund is not reasonably practicable or (2) it is not reasonably practicable for the Fund to fairly determine the net asset value of shares of the Fund;

For any period during which the SEC has, by rule or regulation, deemed that (1) trading shall be restricted or (2) an emergency exists;

For any period that the SEC, may by order permit for your protection; or

For any period during which the Fund as part of a necessary liquidation of the fund, has properly postponed and/or suspended redemption of shares and payment in accordance with federal securities laws.

Rule 2a‑7 as adopted by the SEC permits a money market fund to impose a liquidity fee upon the sale of fund shares under certain circumstances, however the Funds are all government money market funds that have not elected to be subject to the liquidity fee provision of Rule 2a‑7. Although each Fund attempts to maintain its NAV at $1.00 per share, there can be no assurance that it will be successful, and there can be no assurance that a shareholder will receive $1.00 per share upon any redemption.

The transfer agent may temporarily delay for more than seven days the disbursement of redemption proceeds from the Fund account of a “Specified Adult” (as defined in Financial Industry Regulatory Authority, Inc. (“FINRA”) Rule 2165) based on a reasonable belief that financial exploitation of the Specified Adult has occurred, is occurring, has been attempted, or will be attempted, subject to certain conditions.

Under normal circumstances, the Funds expect to meet redemption requests by using cash or cash equivalents in its portfolio and/or selling portfolio assets to generate cash. The Funds also may pay redemption proceeds using cash obtained through borrowing arrangements that may be available from time to time.

A Fund may pay all or a portion of your redemption proceeds by giving you securities (for example, if the Fund reasonably believes that a cash redemption may have a substantial impact on the Fund and its remaining shareholders). A redemption is generally a taxable event for shareholders, regardless of whether the redemption is satisfied in cash or in kind. You may pay transaction costs and/or taxes to dispose of the securities, and you may receive less for them than the price at which they were valued for purposes of the redemption. In addition, you will be subject to the market risks associated with such securities until such time as you choose to dispose of the security.

During periods of deteriorating or stressed market conditions or during extraordinary or emergency circumstances, a Fund may be more likely to pay redemption proceeds with cash obtained through short-term borrowing arrangements (if available) or by giving you securities.

A request for a partial redemption by an investor whose account balance is below the minimum amount or a request for partial redemption by an investor that would bring the account below the minimum amount may be treated as a request for a complete redemption of the account. These minimums may be different for investments made through certain financial intermediaries as deter- mined by their policies and may be waived in the Adviser’s discretion. The Funds reserve the right to modify minimum account requirements at any time with or without prior notice. The Funds also reserve the right to involuntarily redeem an investor’s account if the investor’s account balance falls below the applicable minimum amount due to transaction activity.

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The sub‑section titled “Exchanging Shares” in the section of the Prospectuses titled “Shareholder Information” is hereby deleted and replaced with the following.

Exchanging Shares

An exchange occurs when you use the proceeds from the redemption of shares of a Fund in the State Street Institutional Investment Trust to simultaneously purchase shares of a different Fund in the State Street Institutional Investment Trust. Exchanges may be made between different Funds within the same class (i.e., Institutional Class shares for Institutional Class shares; Bancroft Capital Class shares for Bancroft Capital Class shares). The account holding the original shares must be registered in the same name as the account holding the new shares received in the exchange. You may make exchange requests by telephone, or by mail. See Purchasing Shares and Redeeming Shares. Exchanges are subject to the terms applicable to the purchases of the fund into which you are exchanging. Exchange privileges may not be available for mutual funds advised by SSGA FM (the “State Street Funds”) and may be suspended or rejected.

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The eighth paragraph of the section of the Prospectuses titled “Dividends, Distributions and Tax Considerations” is hereby deleted.

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The first paragraph of the sub‑section titled “Third-Party Transactions” in the section titled “Financial Intermediary Arrangements – Payments to Financial Intermediaries” is hereby deleted and replaced with the following.

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Third-Party Transactions. The State Street Funds have authorized certain Financial Intermediaries to accept purchase, redemption and exchange orders on the State Street Funds’ behalf. Orders received for a State Street Fund by a Financial Intermediary that has been authorized to accept orders on the Fund’s behalf (or other Financial Intermediaries designated by the Financial Intermediary) will be deemed accepted by the Fund at the time they are received by the Financial Intermediary and will be priced based on the Fund’s next NAV determination as long as the Financial Intermediary transmits the order in good form and in a timely manner to the applicable State Street Fund(s). The Financial Intermediary is responsible for transmitting your orders and associated funds in good form and in a timely manner to the applicable State Street Fund(s). The State Street Funds will not be responsible for delays by the Financial Intermediary in transmitting your orders, including timely transfer of payment, to a Fund.

SAIs

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The sub‑section titled “Additional Investments and Risks” is hereby revised to remove the following instruments, techniques, and risks for the Fund:

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Eurodollar Certificates of Deposit (“ECDs”), Eurodollar Time Deposits (“ETDs”) and Yankee Certificates of Deposit (“YCDs”)

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Investment Grade Bonds

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Other Asset Backed Securities

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Private Placements and Restricted Securities

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Variable Amount Master Demand Notes

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“Recent Money Market Fund Regulatory Reforms” in the sub‑section titled “Additional Investments and Risks” is hereby deleted and replaced with the following:

Recent Money Market Regulatory Reforms

On July 12, 2023, the SEC adopted amendments to money market fund regulation (“Money Market Fund Reform”) that increase the daily liquid asset requirements from 10% to 25% and increase the weekly liquid asset requirements from 30% to 50%. Money Market Fund Reform permits government money market funds (such as the Funds), that are experiencing a gross negative yield as a result of negative interest rates, to either convert from a stable share price to a floating share price or reduce the number of shares outstanding (through a reverse stock split) to maintain a stable net asset value per share, subject to certain Board determinations and disclosures to investors. Money Market Fund Reform, among other things, also imposes additional reporting requirements on money market funds.

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The paragraph in the sub‑section titled “ILR Fund” in the section titled “Pricing of Shares” is hereby deleted.

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The heading “Treasury Fund, Treasury Plus Fund, Treasury Obligations Fund and U.S. Government Fund” in the section titled “Pricing of Shares” is hereby deleted.

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The heading “Negative Interest Rate Environments - Treasury Fund, Treasury Plus Fund, Treasury Obligations Fund and U.S. Government Fund” in the section titled “Pricing of Shares” are hereby deleted and replaced with the heading “Negative Interest Rate Environments”.

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The sub‑section “Redemptions and Exchanges” in the section titled “Taxation of the Funds” is hereby deleted and replaced with the following:

Redemptions and Exchanges

Redemptions and exchanges of each Fund’s shares are taxable events and, accordingly, shareholders may realize gain or loss on these transactions to the extent the NAV of Fund Shares varies from a shareholder’s tax basis in such shares. In general, any gain or loss realized upon a taxable disposition of shares will be treated as long-term capital gain or loss if the shares have been held for more than 12 months. Otherwise, the gain or loss on the taxable disposition of Fund shares will be treated as short-term capital gain or loss. However, any loss realized upon a taxable disposition of Fund shares held by a shareholder for six months or less will be treated as long-term, rather than short-term, to the extent of any Capital Gain Dividends received (or deemed received) by the shareholder with respect to the shares.

The IRS permits a simplified method of accounting for gains and losses realized upon the disposition of shares of a RIC that is a money market fund. Very generally, rather than realizing gain or loss upon each redemption of a share, a shareholder of a Fund using such method of accounting will recognize gain or loss with respect to such a Fund’s shares for a given computation period (the shareholder’s taxable year or shorter period selected by the shareholder) equal to the value of all the Fund shares held by the shareholder on the last day of the computation period, less the value of all Fund shares held by the shareholder on the last day of the preceding computation period, less the shareholder’s net investment in the Fund (generally, purchases minus redemptions) made during the computation period. Additionally, any loss realized on a sale of shares of the Fund will not be disallowed under “wash sale” rules to the extent the Fund qualifies as a “money market fund” under the 1940 Act. Shareholders of a Fund are urged to consult their own tax advisors regarding their investment in the Fund.

PLEASE RETAIN THIS SUPPLEMENT FOR FUTURE REFERENCE

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