Form 497 STATE STREET INSTITUTION

State Street Institutional Funds

(the “Trust”)

SUPPLEMENT DATED MAY 15, 2018 TO THE

STATEMENT OF ADDITIONAL INFORMATION (“SAI”),

DATED JANUARY 31, 2018

Effective immediately, the Trust’s SAI is revised to add the following as a new sub-

section, immediately preceding the sub-section entitled “Emerging Markets”

within the section entitled “INVESTMENT STRATEGIES AND RISKS AND

PORTFOLIO HOLDINGS”:

Risks Associated with Investment in China A Shares

Special Risk Considerations of Investing in China. Certain Funds are expected to invest to a limited extent in the securities of Chinese companies, normally restricted to residents of the People’s Republic of China (commonly known as “A Shares” or “China A Shares”). Those Funds would invest in China A Shares through the Shanghai-Hong Kong Stock Connect and Shenzhen-Hong Kong Stock Connect programs (each a “Stock Connect” and together the “Stock Connects”) or other channels. Investing in securities of Chinese issuers, including by investing in China A Shares, involves certain risks and considerations not typically associated with investing in securities of U.S. issuers, including, among others, (i) more frequent (and potentially widespread) trading suspensions and government interventions with respect to Chinese issuers, resulting in a lack of liquidity and in price volatility, (ii) currency revaluations and other currency exchange rate fluctuations or blockage, (iii) the nature and extent of intervention by the Chinese government in the Chinese securities markets, whether such intervention will continue and the impact of such intervention or its discontinuation, (iv) the risk of nationalization or expropriation of assets, (v) the risk that the Chinese government may decide not to continue to support economic reform programs, (vi) limitations on the use of brokers, (vii) potentially higher rates of inflation, (viii) the unavailability of consistently-reliable economic data, (ix) the relatively small size and absence of operating history of many Chinese companies, (x) accounting, auditing and financial reporting standards in China are different from U.S. standards and, therefore, disclosure of certain material information may not be available, (xi) greater political, economic, social, legal and tax-related uncertainty, (xii) higher market volatility caused by any potential regional territorial conflicts or natural disasters, (xiii) higher dependence on exports and international trade, (xiv) the risk of increased trade tariffs, embargoes and other trade limitations, (xv) restrictions on foreign ownership, and (xvi) custody risks associated with investing through programs to access Chinese securities. Significant portions of the Chinese securities markets may become rapidly illiquid, as Chinese issuers have the ability to suspend the trading of their equity securities, and have shown a willingness to exercise that option in response to market volatility and other events. The liquidity of Chinese securities may shrink or disappear suddenly and without warning as a result of adverse economic, market or political events, or adverse investor perceptions, whether or not accurate. Reduced liquidity may make an investment in China A Shares difficult to value accurately.

Stock Connect Investing Risk. China A Shares are equity securities of issuers incorporated in mainland China that are denominated and currently traded in Renminbi on the Shanghai or Shenzhen Stock Exchanges. Subject to minor exceptions, under current regulations in China, foreign investors, such as the Funds, can invest in China A Shares only (i) through certain institutional investors that have obtained a license and quota from the Chinese regulators or (ii) through the Hong Kong-Shanghai Stock Connect or Shenzhen-Hong Kong Stock Connect programs. A Fund may invest in A Shares listed and traded on the Shanghai Stock Exchange or Shenzhen Stock Exchange through the Stock Connect program, or on such other stock exchanges in China which participate in the Stock Connect program from time to time. A Fund’s investments in Stock Connect A Shares are generally subject to Chinese securities regulations and listing rules, among other restrictions that may affect a Fund’s investments and returns, including daily limits on net purchases and transfer restrictions. In addition, the Stock Connect program’s trading, clearance and settlement procedures are relatively untested in China, which could pose risks to a Fund. While overseas investors currently are exempt from paying capital gains or value added taxes on income and gains from investments in Stock Connect A Shares, these Chinese tax rules could be changed, which could result in unexpected tax liabilities for a Fund.

The Stock Connect program will only operate on days when both the Chinese and Hong Kong markets are open for trading and when banks in both markets are open on the corresponding settlement days. There may be occasions when a Fund may be subject

to the risk of price fluctuations of China A Shares during the time when the Stock Connect program is not trading. Because of the way in which China A Shares are held in Stock Connect, a Fund may not be able to exercise the rights of a shareholder and may be limited in its ability to pursue claims against the issuer of a security, and may suffer losses in the event the depository of the Shanghai or Shenzhen Stock Exchanges becomes insolvent. Only certain China A Shares are eligible to be accessed through the Stock Connect program. Such securities may lose their eligibility at any time, in which case they presumably could be sold but could no longer be purchased through the Stock Connect program. The Stock Connect program is a relatively new program. Further developments are likely and there can be no assurance as to the program’s continued existence or whether future developments regarding the program may restrict or adversely affect the Fund’s investments or returns. In addition, the application and interpretation of the laws and regulations of Hong Kong and China, and the rules, policies or guidelines published or applied by relevant regulators and exchanges in respect of the Stock Connect program are uncertain, and they may have a detrimental effect on a Fund’s investments and returns.

PLEASE RETAIN THIS SUPPLEMENT FOR FUTURE REFERENCE

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