Prospectus by Investment Company - Form 497

SEI INSTITUTIONAL INVESTMENTS TRUST

Core Fixed Income Fund
(the "Fund")

Supplement Dated September 30, 2024
to the Fund's Summary Prospectus and Prospectus, each dated September 30, 2024

This Supplement provides new and additional information beyond that contained in the Summary Prospectus and Prospectus and should be read in conjunction with such Summary Prospectus and Prospectus.

SEI Investments Management Corporation and the Board of Trustees have previously determined to remove Western Asset Management Company, LLC and Western Asset Management Company Limited as sub-advisers to the Fund, and to reallocate assets currently under their management to the Fund's existing sub-advisers. On or about October 31, 2024 (the "Effective Date"), the reallocation will be completed.

Therefore, from the date of this Supplement until or around the Effective Date, the Summary Prospectus and Prospectus are hereby amended and supplemented to reflect the following changes to the Fund.

Change in Portfolio Management of the Fund

In the Summary Prospectus and Prospectus, in the Fund Summary for the Fund, under the heading titled "Management," in the chart under the sub-heading titled "Sub-Advisers and Portfolio Managers," the following text is hereby added in the appropriate alphabetical order thereof.

Sub-Adviser		Portfolio Manager		Experience with the Fund		Title with Sub-Adviser
Western Asset Management Company, LLC		Michael C. Buchanan, CFA		Since 2024		Chief Investment Officer
		Mark Lindbloom		Since 2005		Portfolio Manager
		Julien Scholnick, CFA		Since 2020		Portfolio Manager
		Frederick R. Marki, CFA		Since 2020		Portfolio Manager
Western Asset Management Company Limited		Michael C. Buchanan, CFA		Since 2024		Chief Investment Officer

In addition, in the Prospectus, under the section titled "Sub-Advisers," under the sub-section titled "Sub-Advisers and Portfolio Managers," under the heading titled "Core Fixed Income Fund," the following text is hereby added in the appropriate alphabetical order thereof:

Western Asset Management Company, LLC: Western Asset Management Company, LLC (Western Asset), located at 385 East Colorado Boulevard, Pasadena, California 91101, serves as a Sub-Adviser to the Core Fixed Income Fund. A team of investment professionals led by Chief Investment Officer Michael C. Buchanan, CFA and Portfolio Managers Mark S. Lindbloom, Julien Scholnick, CFA, and Frederick R. Marki, CFA, manages the portion of the Core Fixed Income Fund's assets allocated to Western Asset. Mr. Buchanan is responsible for the strategic oversight of the investments and for supervising the operations of the various sector specialist teams dedicated to the specific asset classes. Messrs. Lindbloom, Scholnick, and Marki are responsible for portfolio structure, including sector allocation, duration weighting and term structure decisions. Mr. Buchanan joined Western Asset as a Portfolio Manager in 2005. Mr. Lindbloom joined Western Asset as Portfolio Manager in 2005. Mr. Scholnick joined Western Asset as a Portfolio Manager in 2003. Mr. Marki joined Western Asset as a Portfolio Manager in 2005.

Western Asset Management Company Limited: Western Asset Management Company Limited (Western Asset Limited), located at 10 Exchange Square, Primrose Street, London EC2A 2EN, United Kingdom, serves as a Sub-Adviser to the Core Fixed Income Fund. Chief Investment Officer Michael C. Buchanan, CFA manages the portion of the Core Fixed Income Fund's assets allocated to Western Asset Limited. Mr. Buchanan is responsible for the strategic oversight of the investments and for supervising the operations of the various sector specialist teams dedicated to the specific asset classes. Mr. Buchanan joined Western Asset as a Portfolio Manager in 2005.

There are no other changes to the Summary Prospectus or Prospectus.

PLEASE RETAIN THIS SUPPLEMENT FOR FUTURE REFERENCE

SEI-F-1561 (9/24)

SEI INSTITUTIONAL INVESTMENTS TRUST

Core Fixed Income Fund
(the "Fund")

Supplement Dated September 30, 2024
to the Statement of Additional Information ("SAI") dated September 30, 2024

This Supplement provides new and additional information beyond that contained in the SAI and should be read in conjunction with such SAI.

Change in Portfolio Management of the Fund

SEI Investments Management Corporation and the Board of Trustees have previously determined to remove Western Asset Management Company, LLC and Western Asset Management Company Limited as sub-advisers to the Fund, and to reallocate assets currently under their management to the Fund's existing sub-advisers. On or about October 31, 2024 (the "Effective Date"), the reallocation will be completed.

Therefore, from the date of this Supplement until or around the Effective Date, the SAI is hereby amended and supplemented to reflect the following changes to the Fund.

On the cover page of the SAI, "Western Asset Management Company, LLC" and "Western Asset Management Company Limited" are hereby added in the appropriate alphabetical order thereof.

Under the section titled "The Adviser and Sub-Advisers," under the heading titled "The Sub-Advisers," the following text is hereby added in the appropriate alphabetical order thereof:

WESTERN ASSET MANAGEMENT COMPANY, LLC-Western Asset Management Company, LLC ("Western Asset") serves as a Sub-Adviser to a portion of the assets of the Core Fixed Income Fund. Western Asset operates as a group of coordinated sister companies located in various jurisdictions, and the Firm is headquartered in Pasadena, California. Each Western Asset entity ultimately is a wholly-owned subsidiary of Franklin Resources, Inc. [NYSE: BEN], a global investment management organization with subsidiaries operating as Franklin Templeton in over 165 countries. Western Asset was founded in 1971 and specializes in the management of fixed income funds.

WESTERN ASSET MANAGEMENT COMPANY LIMITED-Western Asset Management Company Limited ("Western Asset Limited") serves as a Sub-Adviser to a portion of the assets of the Core Fixed Income Fund. Western Asset operates as a group of coordinated sister companies located in various jurisdictions, and the Firm is headquartered in Pasadena, California. Each Western Asset entity ultimately is a wholly-owned subsidiary of Franklin Resources, Inc. [NYSE: BEN], a global investment management organization with subsidiaries operating as Franklin Templeton in over 165 countries.

In addition, under the same section, under the heading titled "Portfolio Management," the following text is hereby added in the appropriate alphabetical order thereof:

Western Asset

Compensation. SIMC pays Western Asset a fee based on the assets under management of the Core Fixed Income Fund as set forth in an investment sub-advisory agreement between Western Asset and SIMC. Western Asset pays its investment professionals out of its total revenues and other resources, including the sub-advisory fees earned with respect to the Core Fixed Income Fund. The following information relates to the period ended June 30, 2024.

At Western Asset, one compensation methodology covers all products and functional areas, including portfolio managers. Western Asset's philosophy is to reward its employees through total compensation. Total compensation is reflective of the external market value for skills, experience, ability to produce results and the performance of one's group and Western Asset as a whole.

Discretionary bonuses make up the variable component of total compensation. These are structured to reward sector specialists for contributions to Western Asset as well as relative performance of their specific portfolios/product and are determined by the professional's job function and performance as measured by a formal review process.

For portfolio managers, the formal review process includes a thorough review of portfolios they were assigned to lead or with which they were otherwise involved and includes not only investment performance, but maintaining a detailed knowledge of client portfolio objectives and guidelines, monitoring of risks and performance for adherence to these parameters, execution of asset allocation consistent with current firm and portfolio strategy and communication with clients. In reviewing investment performance, one-, three- and five-year annualized returns are measured against appropriate market peer groups and to each fund's benchmark index.

Ownership of Fund Shares. As of June 30, 2024, Western Asset's portfolio managers did not beneficially own any shares of the Core Fixed Income Fund.

Other Accounts. As of June 30, 2024, in addition to the Core Fixed Income Fund, Western Asset's portfolio managers were responsible for the day-to-day management of certain other accounts, as follows:

		Registered Investment Companies		Other Pooled Investment Vehicles		Other Accounts
Portfolio Manager		Number of Accounts		Total Assets (in millions)		Number of Accounts		Total Assets (in millions)		Number of Accounts		Total Assets (in millions)
Michael C. Buchanan, CFA			75			$	111,692				286			$	66,016				570			$	173,630
			0			$	0				22	*		$	2,556				20	*		$	11,172
Mark S. Lindbloom			29			$	44,192				38			$	15,324				203			$	60,953
			0			$	0				0			$	0				4	*		$	1,734
Julien Scholnick, CFA			27			$	43,981				22			$	11,651				198			$	70,603
			0			$	0				0			$	0				4	*		$	1,734
Frederick R. Marki, CFA			27			$	42,145				25			$	12,611				189			$	58,162
			0			$	0				0			$	0				4	*		$	1,734

* These accounts, which are a subset of the accounts in the preceding row, are subject to a performance-based advisory fee.

Note:The numbers above reflect the overall number of portfolios managed by Western Asset. Mr. Buchanan is involved in the management of all Western Asset's portfolios, but is not solely responsible for particular portfolios. Western Asset's investment discipline emphasizes a team approach that combines the efforts of groups of specialists working in different market sectors. The individuals that have been identified are responsible for overseeing implementation of Western Asset's overall investment ideas and coordinating the work of the various sector teams. This structure ensures that client portfolios benefit from a consensus that draws on the expertise of all team members.

Conflicts of Interest. Potential conflicts of interest may arise in connection with the management of multiple accounts (including accounts managed in a personal capacity). These could include potential conflicts of interest related to the knowledge and timing of the Core Fixed Income Fund's trades, investment opportunities and broker selection. Portfolio managers may be privy to the size, timing and possible market impact of the Core Fixed Income Fund's trades.

It is possible that an investment opportunity may be suitable for both the Core Fixed Income Fund and other accounts managed by a portfolio manager, but may not be available in sufficient quantities for both the Core Fixed Income Fund and the other accounts to participate fully. Similarly, there may be limited opportunity to sell an investment held by the Core Fixed Income Fund and another account. A conflict may arise where the portfolio manager may have an incentive to treat an account preferentially as compared to the Core Fixed Income Fund because the account pays a performance-based fee or the portfolio manager, Western Asset or an affiliate has an interest in the account. Western Asset has adopted procedures to allocate portfolio transactions and investment opportunities across multiple client accounts on a fair and equitable basis over time. All eligible accounts that can participate in a trade share the same price on a pro-rata allocation basis to ensure that no conflict of interest occurs. Trades are allocated among similarly managed accounts to maintain consistency of portfolio strategy, taking into account cash availability, investment restrictions and guidelines and portfolio composition versus strategy.

With respect to securities transactions for the Core Fixed Income Fund, Western Asset determines which broker or dealer to use to execute each order, consistent with their duty to seek best execution of the transaction. However, with respect to certain other accounts (such as pooled investment vehicles that are not registered investment companies and other accounts managed for organizations and individuals), Western Asset may be limited by the client with respect to the selection of brokers or dealers or may be instructed to direct trades through a particular broker or dealer. In these cases, trades for the Core Fixed Income Fund in a particular security may be placed separately from, rather than aggregated with, such other accounts. Having separate transactions with respect to a security may temporarily affect the market price of the security, the execution of the transaction or both, to the possible detriment of the Core Fixed Income Fund or the other accounts involved. Additionally, the management of multiple portfolios and/or other accounts may result in a portfolio manager devoting unequal time and attention to the management of each portfolio and/or other account.

It is theoretically possible that portfolio managers could use information to the advantage of other accounts they manage and to the possible detriment of the Core Fixed Income Fund. For example, a portfolio manager could short sell a security for an account immediately prior to Core Fixed Income Fund's sale of that security. To address this conflict, Western Asset has adopted procedures for reviewing and comparing selected trades of alternative investment accounts (which may make directional trades such as short sales) with long only accounts (which include the Core Fixed Income Fund) for timing and pattern related issues. Trading decisions for alternative investment and long only accounts may not be identical even though the same portfolio manager may manage both types of accounts. Whether Western Asset allocates a particular investment opportunity to only alternative investment accounts or to alternative investment and long only accounts will depend on the investment strategy being implemented. If, under the circumstances, an investment opportunity is appropriate for both its alternative investment and long only accounts, then it will be allocated to both on a pro-rata basis.

A portfolio manager may also face other potential conflicts of interest in managing the Core Fixed Income Fund, and the description above is not a complete description of every conflict of interest that could arise in managing both the Core Fixed Income Fund and the other accounts listed above.

Western Asset Limited

Compensation. SIMC pays Western Asset Limited a fee based on the assets under management of the Core Fixed Income Fund as set forth in an investment sub-advisory agreement between Western Asset Limited and SIMC. Western Asset Limited pays its investment professionals out of its total revenues and other resources, including the sub-advisory fees earned with respect to the Core Fixed Income Fund. The following information relates to the period ended June 30, 2024.

At Western Asset Limited, one compensation methodology covers all products and functional areas, including portfolio managers. Western Asset Limited's philosophy is to reward its employees through total compensation. Total compensation is reflective of the external market value for skills, experience, ability to produce results and the performance of one's group and Western Asset Limited as a whole.

Discretionary bonuses make up the variable component of total compensation. These are structured to reward sector specialists for contributions to Western Asset Limited as well as relative performance of their specific portfolios/product and are determined by the professional's job function and performance as measured by a formal review process.

For portfolio managers, the formal review process includes a thorough review of portfolios they were assigned to lead or with which they were otherwise involved and includes not only investment performance, but maintaining a detailed knowledge of client portfolio objectives and guidelines, monitoring of risks and performance for adherence to these parameters, execution of asset allocation consistent with current firm and portfolio strategy and communication with clients. In reviewing investment performance, one-, three- and five-year annualized returns are measured against appropriate market peer groups and to each fund's benchmark index.

Ownership of Fund Shares. As of June 30, 2024, Western Asset Limited's portfolio manager did not beneficially own any shares of the Core Fixed Income Fund.

Other Accounts. As of June 30, 2024, in addition to the Core Fixed Income Fund, Western Asset Limited's portfolio manager was responsible for the day-to-day management of certain other accounts, as follows:

		Registered Investment Companies		Other Pooled Investment Vehicles		Other Accounts
Portfolio Manager		Number of Accounts		Total Assets (in millions)		Number of Accounts		Total Assets (in millions)		Number of Accounts		Total Assets (in millions)
Michael C. Buchanan, CFA			75			$	111,692				286			$	66,016				570			$	173,630
			0			$	0				22	*		$	2,556				20	*		$	11,172

* These accounts, which are a subset of the accounts in the preceding row, are subject to a performance-based advisory fee.

Conflicts of Interest. Potential conflicts of interest may arise in connection with the management of multiple accounts (including accounts managed in a personal capacity). These could include potential conflicts of interest related to the knowledge and timing of the Core Fixed Income Fund's trades, investment opportunities and broker selection. Portfolio managers may be privy to the size, timing and possible market impact of the Core Fixed Income Fund's trades.

Western Asset Limited has adopted compliance policies and procedures to address a wide range of potential conflicts of interest that could directly impact client portfolios. For example, potential conflicts of interest may arise in connection with the management of multiple portfolios (including portfolios managed in a personal capacity). These could include potential conflicts of interest related to the knowledge and timing of a portfolio's trades, investment opportunities and broker selection. Portfolio managers are privy to the size, timing and possible market impact of a portfolio's trades.

It is possible that an investment opportunity may be suitable for both a portfolio and other accounts managed by a portfolio manager, but may not be available in sufficient quantities for both the portfolio and the other accounts to participate fully. Similarly, there may be limited opportunity to sell an investment held by a portfolio and another account. A conflict may arise where the portfolio manager may have an incentive to treat an account preferentially as compared to a portfolio because the account pays a performance-based fee or the portfolio manager, Western Asset Limited or an affiliate has an interest in the account. Western Asset Limited has adopted procedures for allocation of portfolio transactions and investment opportunities across multiple client accounts on a fair and equitable basis over time. All eligible accounts that can participate in a trade share the same price on a pro-rata allocation basis to ensure that no conflict of interest occurs. Trades are allocated among similarly managed accounts to maintain consistency of portfolio strategy, taking into account cash availability, investment restrictions and guidelines and portfolio composition versus strategy.

With respect to securities transactions, Western Asset Limited determines which broker or dealer to use to execute each order, consistent with its duty to seek best execution of the transaction. However, with respect to certain other accounts (such as pooled investment vehicles that are not registered investment companies and other accounts managed for organizations and individuals), Western Asset Limited may be limited by the client with respect to the selection of brokers or dealers or may be instructed to direct trades through a particular broker or dealer. In these cases, trades for a portfolio in a particular security may be placed separately from, rather than aggregated with, such other accounts. Having separate transactions with respect to a security may temporarily affect the market price of the security or the execution of the transaction, or both, to the possible detriment of a portfolio or the other account(s) involved. Additionally, the management of multiple portfolios and/or other accounts may result in a portfolio manager devoting unequal

time and attention to the management of each portfolio and/or other account. Western Asset Limited's team approach to portfolio management and block trading approach works to limit this potential risk.

Western Asset Limited also maintains a gift and entertainment policy to address the potential for a business contact to give gifts or host entertainment events that may influence the business judgment of an employee. Employees are permitted to retain gifts of only a nominal value and are required to make reimbursement for entertainment events above a certain value. All gifts (except those of a de minimus value) and entertainment events that are given or sponsored by a business contact are required to be reported in a gift and entertainment log, which is reviewed on a regular basis for possible issues.

Employees of Western Asset Limited have access to transactions and holdings information regarding client accounts and Western Asset Limited's overall trading activities. This information represents a potential conflict of interest because employees may take advantage of this information as they trade in their personal accounts. Accordingly, Western Asset Limited maintains a Code of Ethics that is compliant with Rule 17j-1 and Rule 204A-1 to address personal trading. In addition, the Code of Ethics seeks to establish broader principles of good conduct and fiduciary responsibility in all aspects of Western Asset Limited's business. The Code of Ethics is administered by the Legal & Compliance Department and monitored through Western Asset Limited's compliance monitoring program.

Western Asset Limited may also face other potential conflicts of interest with respect to managing client assets, and the description above is not a complete description of every conflict of interest that could arise. Western Asset Limited also maintains a compliance monitoring program and engages independent auditors to conduct a SSAE 16/ISAE 3402 audit on an annual basis. These steps help to ensure that potential conflicts of interest have been addressed.

There are no other changes to the SAI.

PLEASE RETAIN THIS SUPPLEMENT FOR FUTURE REFERENCE

SEI-F-1562 (9/24)