Click here to view the Fund's Summary Prospectus Click here to view the Fund's Prospectus Pioneer Core Equity Fund Pioneer Series Trust XI 60 State Street Boston, Massachusetts 02109 Statement of Additional Information | May 1, 2021 Class A Shares Class C Shares Class K Shares Class R Shares Class Y Shares PIOTX PCOTX PCEKX CERPX PVFYX This statement of additional information is not a prospectus. It should be read in conjunction with the fund’s Class A, Class C, Class K, Class R and Class Y shares prospectus dated May 1, 2021, as supplemented or revised from time to time. A copy of the prospectus can be obtained free of charge by calling the fund at 1-800-225-6292 or by written request to the fund at 60 State Street, Boston, Massachusetts 02109. You can also obtain a copy of the prospectus from our website at: amundi.com/US. The fund’s financial statements for the fiscal year ended December 31, 2020, including the independent registered public accounting firm’s report thereon, are incorporated into this statement of additional information by reference. Contents 1. Fundhistory................................................................. 1 2. Investmentpolicies,risksandrestrictions....................................... 1 3. Trusteesandofficers......................................................... 28 4. Investmentadviser........................................................... 38 5. Principal underwriter and distribution plan ......................................40 6. Shareholder servicing/transfer agent ........................................... 42 7. Custodianandsub-administrator...............................................43 8. Independentregisteredpublicaccountingfirm..................................43 9. Portfoliomanagement........................................................43 10. Portfoliotransactions.........................................................46 11. Description of shares ......................................................... 47 12. Sales charges ................................................................50 13. Redeeming shares............................................................ 55 14. Telephone and online transactions.............................................. 56 15. Pricingofshares............................................................. 57 16. Taxstatus................................................................... 58 17. Financial statements.......................................................... 65 18. Annualfee,expenseandotherinformation...................................... 65 19. Appendix A — Description of short-term debt, corporate bond and preferred stock ratings......................................................................70 20. AppendixB—Proxyvotingpoliciesandprocedures..............................74 1. Fund history The fund is a diversified open-end management investment company. The fund is a series of Pioneer Series Trust XI (the “Trust”). The Trust was originally organized as a Massachusetts corporation on March 18, 1969 and then reorganized as a Massachusetts business trust on February 15, 1985. It was reorganized as a Delaware statutory trust on May 1, 1996. Prior to July 2, 2013, the Trust was named Pioneer Value Fund. Prior to April 2, 2001, the name of the Trust and the fund was “Pioneer II.” On June 7, 2013, the fund acquired the assets and liabilities of Pioneer Research Fund (the predecessor fund). As a result of the reorganization, the predecessor fund’s performance and financial history became the fund’s performance and financial history. As part of the reorganization, the fund was re-named Pioneer Core Equity Fund. Amundi Asset Management US, Inc. (“Amundi US”) is the fund’s investment adviser. 2. Investment policies, risks and restrictions The prospectus presents the investment objective and the principal investment strategies and risks of the fund. This section supplements the disclosure in the fund’s prospectus and provides additional information on the fund’s investment policies or restrictions. Restrictions or policies stated as a maximum percentage of the fund’s assets are only applied immediately after a portfolio investment to which the policy or restriction is applicable (other than the limitations on borrowing and illiquid securities). Accordingly, any later increase or decrease in a percentage resulting from a change in values, net assets or other circumstances will not be considered in determining whether the investment complies with the fund’s restrictions and policies. Equity securities and related investments Investments in equity securities Equity securities, such as common stock, generally represent an ownership interest in a company. While equity securities have historically generated higher average returns than fixed income securities, equity securities have also experienced significantly more volatility in those returns. An adverse event, such as an unfavorable earnings report, may depress the value of a particular equity security held by the fund. Also, the prices of equity securities, particularly common stocks, are sensitive to general movements in the stock market. A drop in the stock market may depress the price of equity securities held by the fund. Warrants and stock purchase rights The fund may invest in warrants, which are securities permitting, but not obligating, their holder to subscribe for other securities. Warrants do not carry with them the right to dividends or voting rights with respect to the securities that they entitle their holders to purchase, and they do not represent any rights in the assets of the issuer. The fund may also invest in stock purchase rights. Stock purchase rights are instruments, frequently distributed to an issuer’s shareholders as a dividend, that entitle the holder to purchase a specific number of shares of common stock on a specific date or during a specific period of time. The exercise price on the rights is normally at a discount from market value of the common stock at the time of distribution. The rights do not carry with them the right to dividends or to vote and may or may not be transferable. Stock purchase rights are frequently used outside of the United States as a means of raising additional capital from an issuer’s current shareholders. As a result, an investment in warrants or stock purchase rights may be considered more speculative than certain other types of investments. In addition, the value of a warrant or a stock purchase right does not necessarily change with the value of the underlying securities, and warrants and stock purchase rights expire worthless if they are not exercised on or prior to their expiration date. Preferred shares The fund may invest in preferred shares. Preferred shares are equity securities, but they have many characteristics of fixed income securities, such as a fixed dividend payment rate and/or a liquidity preference over the issuer’s common shares. However, because preferred shares are equity securities, they may be more susceptible to risks traditionally associated with equity investments than the fund's fixed income securities. 1 Preferred stocks may differ in many of their provisions. Among the features that differentiate preferred stocks from one another are the dividend rights, which may be cumulative or noncumulative and participating or non-participating, redemption provisions, and voting rights. Such features will establish the income return and may affect the prospects for capital appreciation or risks of capital loss. The market prices of preferred stocks are subject to changes in interest rates and are more sensitive to changes in an issuer's creditworthiness than are the prices of debt securities. Shareholders of preferred stock may suffer a loss of value if dividends are not paid. Under ordinary circumstances, preferred stock does not carry voting rights. Investments in initial public offerings Companies involved in initial public offering (IPOs) generally have limited operating histories, and prospects for future profitability are uncertain. The market for IPO issuers has been volatile, and share prices of newly public companies have fluctuated significantly over short periods of time. Further, stocks of newly-public companies may decline shortly after the IPO. There is no assurance that the fund will have access to IPOs. The purchase of IPO shares may involve high transaction costs. Because of the price volatility of IPO shares, the fund may choose to hold IPO shares for a very short period of time. This may increase the turnover of the portfolio and may lead to increased expenses to the fund, such as commissions and transaction costs. The market for IPO shares can be speculative and/or inactive for extended periods of time. There may be only a limited number of shares available for trading. The limited number of shares available for trading in some IPOs may also make it more difficult for the fund to buy or sell significant amounts of shares without an unfavorable impact on prevailing prices. Non-U.S. investments Equity securities of non-U.S. issuers The fund may invest in equity securities of non-U.S. issuers, including American Depositary Receipts (“ADRs”), European Depositary Receipts (“EDRs”), Global Depositary Receipts (“GDRs”) and other similar instruments. Debt obligations of non-U.S. governments The fund may invest in all types of debt obligations of non-U.S. governments. An investment in debt obligations of non-U.S. governments and their political subdivisions (sovereign debt) involves special