Eaton Vance Senior Income Trust

This proxy statement is furnished in connection with the solicitation of proxies by the Board of Trustees of Eaton Vance Senior Income Trust. The proxies will be voted at the Annual Meeting of Shareholders ofthe Fund and at any adjournments or postponements thereof (the“Annual Meeting”). The Annual Meeting will be held on Monday, November 2, 2026 at 11:30 a.m. (Eastern Time) at the principal office ofthe Fund, One Post Office Square, Boston, Massachusetts 02109, as discussed further herein. The Annual Meeting will be held for thepurposes set forth in the accompanying notice. This proxy statement and the enclosed proxy card(s) are first being sent or given toshareholders on or about October 9, 2026.

The Board of Trustees ofthe Fund (the “Board” or the “Board of Trustees”) has fixed the close of business on September 18, 2026 as therecord date for the determination of the shareholders entitled to notice of and to vote at the Annual Meeting and any adjournments orpostponements thereof. As of September 18, 2026, there were 18,170,289 common shares of beneficial interest, $0.01 par value per share(“Common Shares”) ofthe Fund outstanding. See “Proxy Solicitation, Tabulation and Voting Requirements” below for additional information. To the knowledge of the Fund, as of September 18, 2026: (i) noshareholder owned 5% or more of the Fund’s outstanding Common Shares, and (ii) the Trustees and executive officers ofthe Fund, individually and as a group, owned beneficially less than 1% of the outstanding Common Shares ofthe Fund. As of September 15, 2026, the Fund redeemed all of its outstanding auction preferred shares. The auction preferred shares were redeemable at the option of the Fund in whole or in part at the liquidation value of $25,000 per share plus accumulated and unpaid dividends.

Shareholders as of the close of business on the record date of September 18, 2026 are entitled to attend and vote at the Annual Meeting. Allproperly executed proxies received prior to the Annual Meeting will be voted at the Annual Meeting. Each proxy will be voted in accordance with its instructions; if no instruction is given, an executed proxy will authorize the persons named onthe proxy card enclosed as proxies, orany of them, to vote FOR the election of each Trustee. An executed proxy delivered tothe Fund is revocable by the person giving it, prior toits exercise, by a signed writing filed with the Fund’s Secretary, by executing and delivering a later dated proxy, or by attending the Annual Meeting and voting the shares at the Annual Meeting. Merely attending the Annual Meeting will not revoke a previously executed proxy. Ifyou hold Fund shares through an intermediary (such as a broker, bank, adviser or custodian), please consult with the intermediary regarding your ability to revoke voting instructions after they have been provided.

If you are a record holder of Fund shares and plan to attend the Annual Meeting, you must show a valid photo identification (such as adriver’s license) to gain admission to the Annual Meeting. Please call 1-800-262-1122 for information on how to obtain directions to be able toattend and vote at the Annual Meeting.

If you hold Fund shares through an intermediary and plan to attend and vote at the Annual Meeting, you will be required to show a valid photo identification and your authority to vote your shares (referred to as a “legal proxy”) to gain admission to the Annual Meeting. Asdescribed above, you must contact your intermediary to obtain a legal proxy for your shares.