Eaton Vance Senior Income Trust : Prospectus

This Prospectus Supplement, together with the accompanying Prospectus, sets forth concisely information about the Trust that you should know before investing. You should read this Prospectus Supplement and the accompanying Prospectus, which contain important information, before deciding whether to invest in our securities. You should retain the accompanying Prospectus and this Prospectus Supplement for future reference. A Statement of Additional Information ("SAI"), dated August 25, 2022 as supplemented from time to time, containing additional information about the Trust, has been filed with the Securities and Exchange Commission (the "SEC") and is incorporated by reference in its entirety into this Prospectus Supplement and the accompanying Prospectus. This Prospectus Supplement, the accompanying Prospectus and the SAI are part of a "shelf" registration statement that we filed with the SEC. This Prospectus Supplement describes the specific details regarding this offering (as defined below), including the method of distribution. If information in this Prospectus Supplement is inconsistent with the accompanying Prospectus or the SAI, you should rely on this Prospectus Supplement. You may request a free copy of the SAI, the table of contents of which is on page 60 of the accompanying Prospectus or a free copy of our annual and semi- annual reports to shareholders, obtain other information or make shareholder inquiries, by calling toll-free1-800-262-1122 or by writing to the Trust at Two International Place, Boston, Massachusetts 02110. The Trust's SAI and annual and semi-annual reports also are available free of charge on our website at http://www.eatonvance.com and on the SEC's website (http://www.sec.gov). You may also obtain these documents, after paying a duplication fee, by electronic request at the following email address: publicinfo@sec.gov.

Our securities do not represent a deposit or obligation of, and are not guaranteed or endorsed by, any bank or other insured depository institution and are not federally insured by the Federal Deposit Insurance Corporation, the Federal Reserve Board or any other government agency.

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TABLE OF CONTENTS

You should rely only on the information contained in, or incorporated by reference into, this Prospectus Supplement and the accompanying Prospectus in making your investment decisions. The Trust has not authorized any person to provide you with different information. If anyone provides you with different or inconsistent information, you should not rely on it. The Trust is not making an offer to sell the securities in any jurisdiction where the offer or sale is not permitted. You should assume that the information in this Prospectus Supplement and the accompanying Prospectus is accurate only as of the dates on their covers. The Trust's business, financial condition and prospects may have changed since the date of its description in this Prospectus Supplement or the date of its description in the accompanying Prospectus.

Until September 19, 2022 (25 days after the date of this Prospectus Supplement), all dealers that buy, sell or trade the Common Shares, whether or not participating in this offering (as defined below), may be required to deliver the Prospectus and this Prospectus Supplement. This requirement is in addition to the dealers' obligation to deliver the Prospectus and this Prospectus Supplement when acting as underwriters and with respect to their unsold allotments or subscriptions.

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CAUTIONARY NOTICE REGARDING FORWARD-LOOKING STATEMENTS

This Prospectus Supplement, the accompanying Prospectus and the SAI contain "forward-looking statements." Forward-looking statements can be identified by the words "may," "will," "intend," "expect," "estimate," "continue," "plan," "anticipate," and similar terms and the negative of such terms. Such forward-looking statements may be contained in this Prospectus Supplement as well as in the accompanying Prospectus. By their nature, all forward-looking statements involve risks and uncertainties, and actual results could differ materially from those contemplated by the forward-looking statements. Several factors that could materially affect our actual results are the performance of the portfolio of securities we hold, the price at which our shares will trade in the public markets and other factors discussed in our periodic filings with the SEC.

Although we believe that the expectations expressed in our forward-looking statements are reasonable, actual results could differ materially from those projected or assumed in our forward-looking statements. Our future financial condition and results of operations, as well as any forward-looking statements, are subject to change and are subject to inherent risks and uncertainties, such as those disclosed in the "Investment Objective, Policies and Risks" section of the accompanying Prospectus. All forward- looking statements contained or incorporated by reference in this Prospectus Supplement or the accompanying Prospectus are made as of the date of this Prospectus Supplement or the accompanying Prospectus, as the case may be. Except for our ongoing obligations under the federal securities laws, we do not intend, and we undertake no obligation, to update any forward-looking statement. The forward-looking statements contained in this Prospectus Supplement, the accompanying Prospectus and the SAI are excluded from the safe harbor protection provided by section 27A of the 1933 Act.

Currently known risk factors that could cause actual results to differ materially from our expectations include, but are not limited to, the factors described in the "Investment Objective, Policies and Risks" section of the accompanying Prospectus. We urge you to review carefully that section for a more detailed discussion of the risks of an investment in our securities.

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Prospectus Supplement Summary

The following summary is qualified in its entirety by reference to the more detailed information included elsewhere in this Prospectus Supplement and in the accompanying Prospectus and in the SAI.

THE TRUST

Eaton Vance Senior Income Trust (the "Trust," "we," or "our") is a diversified, closed-end management investment company, which commenced operations on October 30, 1998. The Trust offers investors the opportunity to receive a high level of current income, through a professionally managed portfolio investing primarily in senior, secured floating-rate loans ("Senior Loans"), which are normally accessible only to financial institutions and large corporate and institutional investors, and are not widely available to individual investors. To the extent consistent with this objective, the Trust may also offer an opportunity for preservation of capital. Investments are based on Eaton Vance Management's ("Eaton Vance" or the "Adviser") internal research and ongoing credit analysis, which is generally not available to individual investors. An investment in the Trust may not be appropriate for all investors. There is no assurance that the Trust will achieve its investment objective.

THE ADVISER

Eaton Vance acts as the Trust's investment adviser. Eaton Vance's principal office is located at Two International Place, Boston, MA 02110. Eaton Vance, its affiliates and predecessor companies have been managing assets of individuals and institutions since 1924 and of investment companies since 1931. Prior to March 1, 2021, Eaton Vance was a wholly owned subsidiary of Eaton Vance Corp. ("EVC").

On March 1, 2021, Morgan Stanley acquired EVC (the "Transaction") and Eaton Vance became an indirect, wholly owned subsidiary of Morgan Stanley. In connection with the closing of the Transaction, the Trust entered into an interim investment advisory agreement (the "Interim Agreement") with Eaton Vance, which took effect on March 1, 2021. The Interim Agreement allowed Eaton Vance to continue to manage the Trust for up to an additional 150 days following the Transaction to provide more time for further proxy solicitation in connection with shareholder approval of a new investment advisory agreement. Compensation payable to Eaton Vance pursuant to the Interim Agreement was required to be held in an interest-bearing escrow account with the Trust's custodian. The Advisory Agreement was approved by Trust shareholders on June 24, 2021.

Morgan Stanley (NYSE: MS), whose principal offices are at 1585 Broadway, New York, New York 10036, is a preeminent global financial services firm engaged in securities trading and brokerage activities, as well as providing investment banking, research and analysis, financing and financial advisory services. As of June 30, 2022, Morgan Stanley's asset management operations had aggregate assets under management of approximately $1.4 trillion.

Under the general supervision of the Trust's Board, the Adviser will carry out the investment and reinvestment of the assets of the Trust, will furnish continuously an investment program with respect to the Trust, will determine which securities should be purchased, sold or exchanged, and will implement such determinations. The Adviser will furnish to the Trust investment advice and office facilities, equipment and personnel for servicing the investments of the Trust. The Adviser will compensate all Trustees and officers of the Trust who are members of the Adviser's organization and who render investment services to the Trust, and will also compensate all other Adviser personnel who provide research and investment services to the Trust. In return for these services, facilities and payments, the Trust, has agreed to pay the Adviser as compensation under the Advisory Agreement a fee computed at an annual rate of 0.72% (0.73% prior to May 1, 2022) of the average weekly gross assets, payable monthly. The annual investment adviser fee rate shall be reduced to the following as of the stated date: May 1, 2023: 0.71%, May 1, 2024: 0.70%, May 1, 2025: 0.69% and May 1, 2026: 0.55%. Gross assets shall be calculated by deducting accrued liabilities of the Trust except the principal amount of any indebtedness for money borrowed, including debt securities issued by the Trust. Accrued liabilities are expenses incurred in the normal course of operations. During periods in which the Trust is using leverage, the fees paid to Eaton Vance for investment advisory services will be higher than if the Trust did not use leverage because the fees paid will be calculated on the basis of the Trust's gross assets, including proceeds from any borrowings and from the issuance of preferred shares. The Trust is responsible for all expenses not expressly stated to be payable by another party (such as the expenses required to be paid pursuant to an agreement with the investment adviser or administrator).

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