Summary Prospectus by Investment Company - Form 497K

September 30, 2024

SUMMARY PROSPECTUS

SIIT Extended Market Index Fund (SMXAX)

Class A

Before you invest, you may want to review the Fund's Prospectus, which contains information about the Fund and its risks. You can find the Fund's Prospectus and other information about the Fund, including the Fund's Statement of Additional Information and annual and semi-annual reports, online at seic.com/fundprospectuses. You can also get this information at no cost by dialing 1-800-DIAL-SEI. The Fund's Prospectus and Statement of Additional Information, dated September 30, 2024, as may be supplemented from time to time, are incorporated by reference into this Summary Prospectus and may be obtained, free of charge, at the website or phone number noted above.

seic.com

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Investment Goal

Seeks investment results that approximate, as closely as practicable and before expenses, the performance of the Russell Small Cap Completeness Index.

Fees and Expenses

This table describes the fees and expenses that you may pay if you buy, hold, and sell Fund shares. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in this table and examples below.

ANNUAL FUND OPERATING EXPENSES

EXAMPLE

This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund 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 Fund's operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

PORTFOLIO TURNOVER

The Fund will pay transaction costs, such as commissions, when it buys and sells securities (or "turns over" its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when Fund shares are held in a taxable account. These costs, which are not reflected in annual Fund operating expenses or in the Example, affect the Fund's performance. During the most recent fiscal year, the Fund's portfolio turnover rate was 34% of the average value of its portfolio.

Principal Investment Strategies

The Fund is managed using a passive/indexing investment approach and invests substantially all of its assets in securities (mostly common stocks) of companies that are included (at the time of purchase) in the Russell Small Cap Completeness Index (the Index). As of July 31, 2024, the market capitalization of the companies included in the Index ranged from $17.1 million to $83.3 billion. The Index is composed of securities of the companies included in the Russell 3000 Index (which includes the largest 3,000 U.S. companies), excluding the securities of companies that are constituents of the S&P 500 Index (which includes 500 leading U.S. companies). The Index is constructed to attempt to provide a comprehensive and unbiased barometer of the extended broad market of U.S. equity securities beyond that of the 500 leading U.S. companies included in the S&P 500 Index. The Fund's investment performance will depend on the Fund's tracking of the Index and the performance of the Index. The market capitalization range and the composition of the Index are subject to change.

The Fund's sub-adviser (the Sub-Adviser) selects the Fund's securities under the general supervision of SEI Investments Management Corporation (SIMC or the Adviser), but the Sub-Adviser makes no attempt to "manage" the Fund in the traditional sense (i.e., by using economic, market or financial analyses). Instead, the Fund generally will attempt to invest in securities (including interests of real estate investment trusts (REITs)) composing the Index in approximately the same proportions as they are represented in the Index. The Fund's ability to fully replicate the performance of the Index will depend to some extent on the size and timing of cash flows into and out of the Fund, as well as on the level of the Fund's expenses. In some cases, it may not be possible or practicable to purchase all of the securities composing the Index or to hold them in the same weightings as they are represented in the Index. In those cases, the Fund's Sub-Adviser may employ a sampling or optimization technique to construct the Fund's portfolio. In seeking to replicate the performance of the Index, the Fund may also invest in exchange-traded funds (ETFs) and REITs that are not constituents of the Index.

The Sub-Adviser may sell an investment if the merit of the investment has been substantially impaired by extraordinary events or adverse financial conditions. The Fund may, at times, purchase or sell index futures contracts, or options on those futures, or engage in forward or swap transactions in lieu of investing directly in the securities making up the Index or to enhance the Fund's

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replication of the Index's return. In addition, for liquidity purposes, the Fund may invest in securities that are not included in the Index, cash and cash equivalents or money market instruments, such as reverse repurchase agreements and money market funds. The Fund's return may not match the return of the Index.

Principal Risks

Market Risk - The risk that the market value of a security may move up and down, sometimes rapidly and unpredictably. Market risk may affect a single issuer, an industry, a sector or the equity market as a whole. Equity markets may decline significantly in response to adverse issuer, political, regulatory, market, economic or other developments that may cause broad changes in market value, public perceptions concerning these developments, and adverse investor sentiment or publicity. Similarly, environmental and public health risks, such as natural disasters, epidemics, pandemics or widespread fear that such events may occur, may impact markets adversely and cause market volatility in both the short- and long-term.

Small and Medium Capitalization Risk - The small and medium capitalization companies in which the Fund may invest may be more vulnerable to adverse business or economic events than larger, more established companies. In particular, small and medium capitalization companies may have limited product lines, markets and financial resources and may depend upon a relatively small management group. Therefore, small and medium capitalization stocks may be more volatile than those of larger companies. Small and medium capitalization stocks may be traded over-the-counter (OTC). OTC stocks may trade less frequently and in smaller volume than exchange listed stocks and may have more price volatility than that of exchange-listed stocks.

Investment Style Risk - The risk that the Fund's investment approach, which attempts to replicate the performance of the Index, may underperform other segments of the equity markets or the equity markets as a whole. The Fund is also subject to the risk that the securities in which it invests may underperform other segments of the equity markets or the equity markets as a whole.

Tracking Error Risk - The risk that the Fund's performance may vary substantially from the performance of the Index as a result of cash flows, Fund expenses, imperfect correlation between the Fund's investments and the benchmark and other factors.

Sampling Risk - The Fund may not fully replicate the Index and may hold securities not included in the Index. As a result, the Fund may not track the return of the Index as well as it would have if the Fund purchased all of the securities in the Index.

Exchange-Traded Funds Risk - The risks of owning shares of an ETF generally reflect the risks of owning the underlying securities the ETF is designed to track, although lack of liquidity in an ETF could result in its value being more volatile than the underlying portfolio securities. When the Fund invests in an ETF, in addition to directly bearing the expenses associated with its own operations, it will bear a pro rata portion of the ETF's expenses.

REITs Risk - REITs are trusts that invest primarily in commercial real estate or real estate-related loans. The Fund's investments in REITs will be subject to the risks associated with the direct ownership of real estate. Risks commonly associated with the direct ownership of real estate include fluctuations in the value of underlying properties, defaults by borrowers or tenants, changes in interest rates and risks related to general or local economic conditions. Some REITs may have limited diversification and may be subject to risks inherent in financing a limited number of properties.

Large Capitalization Risk - The risk that larger, more established companies may be unable to respond quickly to new competitive challenges such as changes in technology and consumer tastes. Larger companies also may not be able to attain the high growth rates of successful smaller companies.

Derivatives Risk - The Fund's use of futures contracts, forwards contracts, options and swaps is subject to market risk, leverage risk, correlation risk and liquidity risk. Leverage risk and liquidity risk are described below and market risk is described above. Many OTC derivative instruments will not have liquidity beyond the counterparty to the instrument. Correlation risk is the risk that changes in the value of the derivative may not correlate perfectly with the underlying asset, rate or index. The Fund's use of forward contracts, options and swaps is also subject to credit risk and valuation risk. Credit risk is described below. Valuation risk is the risk that the derivative may be difficult to value and/or valued incorrectly. Each of these risks could cause the Fund to lose more than the principal amount invested in a derivative instrument. Some derivatives have the potential for unlimited loss, regardless of the size of the Fund's initial investment. The other parties to certain derivative contracts present the same types of credit risk as issuers of fixed income securities. The Fund's use of derivatives may also increase the amount of taxes payable by shareholders. Both U.S. and non-U.S. regulators have adopted and implemented regulations governing derivatives markets, the ultimate impact of which remains unclear.

Repurchase Agreements and Reverse Repurchase Agreements Risk - In the event of the insolvency of the counterparty to a repurchase agreement or reverse repurchase agreement, recovery of the repurchase price owed to the Fund or, in the case of a reverse repurchase agreement, the securities sold by the Fund, may be delayed. Because reverse repurchase agreements may be considered to be the practical equivalent of borrowing funds, they constitute a form of leverage. If the Fund reinvests the proceeds of a reverse repurchase agreement at a rate lower than the cost of the agreement, entering into the agreement will lower the Fund's yield.

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Money Market Funds - An investment in a money market fund is not a bank deposit and is not insured or guaranteed by any bank, the Federal Deposit Insurance Corporation (FDIC) or any other government agency. Certain money market funds float their NAV while others seek to preserve the value of investments at a stable NAV (typically, $1.00 per share). An investment in a money market fund, even an investment in a fund seeking to maintain a stable NAV per share, is not guaranteed and it is possible for the Fund to lose money by investing in these and other types of money market funds. If the liquidity of a money market fund's portfolio deteriorates below certain levels, the money market fund may suspend redemptions (i.e., impose a redemption gate) and thereby prevent the Fund from selling its investment in the money market fund or impose a fee of up to 2% on amounts the Fund redeems from the money market fund (i.e., impose a liquidity fee). Money market funds and the securities they invest in are subject to comprehensive regulations. The enactment of new legislation or regulations, as well as changes in interpretation and enforcement of current laws, may affect the manner of operation, performance and/or yield of money market funds.

Liquidity Risk - The risk that certain securities may be difficult or impossible to sell at the time and the price that the Fund would like. The Fund may have to lower the price of the security, sell other securities instead or forego an investment opportunity, any of which could have a negative effect on Fund management or performance.

Leverage Risk - The Fund's use of derivatives and repurchase agreements (which effectively constitute a form of borrowing) may result in the Fund's total investment exposure substantially exceeding the value of its portfolio securities and the Fund's investment returns depending substantially on the performance of securities that the Fund may not directly own. The use of leverage can amplify the effects of market volatility on the Fund's share price and may also cause the Fund to liquidate portfolio positions when it would not be advantageous to do so in order to satisfy its obligations. The Fund's use of leverage may result in a heightened risk of investment loss.

Opportunity Risk - The risk of missing out on an investment opportunity because the assets necessary to take advantage of it are tied up in other investments.

Credit Risk - The risk that the issuer of a security or the counterparty to a contract will default or otherwise become unable to honor a financial obligation.

Investing in the Fund involves risk, and there is no guarantee that the Fund will achieve its investment goal. You could lose money on your investment in the Fund, just as you could with other investments. An investment in the Fund is not a deposit of a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.

Performance Information

The bar chart and the performance table below provide some indication of the risks of investing in the Fund by showing changes in the Fund's performance from year to year for the past ten calendar years, and by showing how the Fund's average annual returns for 1, 5 and 10 years, and since the Fund's inception compare with those of a broad measure of market performance. The performance information shown is based on full calendar years. The Fund's past performance (before and after taxes) is not necessarily an indication of how the Fund will perform in the future. For current performance information, please call 1-800-DIAL-SEI.

Average Annual Total Returns (for the periods ended December 31, 2023)

This table compares the Fund's average annual total returns to those of an appropriate broad-based index and an additional index with characteristics relevant to the Fund's investment strategy.

After-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes. Your actual after-tax returns will depend on your tax situation and may differ from those shown.

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After-tax returns shown are not relevant to investors who hold their Fund shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts.

Management

Investment Adviser and Portfolio Managers. SEI Investments Management Corporation

Sub-Adviser and Portfolio Managers.

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Purchase and Sale of Fund Shares

The Fund's minimum investment requirements for Class A Shares are: (a) that you must be an Eligible Investor (i.e., institutions or other SIMC advisory clients that have entered into an investment management agreement with SIMC or institutional investors, employee benefit plans and other similar entities purchasing through approved intermediaries); and (b) that your minimum initial investment must be $100,000, with minimum subsequent investments of $1,000, which may be waived at the discretion of SIMC. You may purchase and redeem shares of the Fund on any day that the New York Stock Exchange (NYSE) is open for business (a Business Day). You may sell your Fund shares by contacting your authorized financial institution or intermediary directly. Authorized financial institutions and intermediaries may redeem Fund shares on behalf of their clients by contacting the Fund's transfer agent (the Transfer Agent) or the Fund's authorized agent, using certain SEI Investments Company (SEI) or third party systems or by calling 1-800-858-7233, as applicable.

Tax Information

The distributions made by the Fund are generally taxable and will be taxed as qualified dividend income, ordinary income or capital gains. If you are investing through a tax-deferred arrangement, such as a 401(k) plan or individual retirement account, you will generally not be subject to federal taxation on Fund distributions until you begin receiving distributions from your tax-deferred arrangement. You should consult your tax advisor regarding the rules governing your tax-deferred arrangement.

Payments to Broker-Dealers and Other Financial Intermediaries

If you purchase Fund shares through a broker-dealer or other financial intermediary, such as a bank, the Fund and its related companies may pay the intermediary for the sale of Fund shares and related services. These payments may create a conflict of interest by influencing the broker-dealer or other intermediary and your salesperson to recommend the Fund over another investment. Ask your salesperson or visit your financial intermediary's website for more information.