NBT BANCORP INC

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 14A

Proxy Statement Pursuant to Section 14(a) of the

Securities Exchange Act of 1934 (Amendment No. )

Filed by Registrant ☑

Filed by Party other than the Registrant ☐

Check the appropriate box:

NBT Bancorp Inc.

(Name of Registrant as Specified in Its Charter)

Payment of Filing Fee (Check the appropriate box):

Notice of 2026 Annual Meeting of Stockholders

NBT Bancorp Inc. (“NBT” or “the Company”) will hold an annual meeting of stockholders on May 19, 2026 at 11:00 a.m. EDT (the “Annual Meeting”). You can attend the 2026 Annual Meeting online and vote your shares electronically. The Annual Meeting will be virtual and conducted through the online means described below.

The Annual Meeting is being held for the following purposes:

We have fixed the close of business on March 23, 2026 as the record date for determining those stockholders of NBT entitled to vote at the Annual Meeting and any adjournments or postponements of the meeting. Only holders of record of NBT common stock at the close of business on that date are entitled to notice of, and to vote at, the Annual Meeting.

By Order of the Board of Directors of

NBT Bancorp Inc.

/s/ Martin A. Dietrich

Martin A. Dietrich

Chairman of the Board of Directors

Norwich, New York

April 6, 2026

Important Notice regarding the Internet availability of Proxy Materials for the Annual Meeting to be held on May 19, 2026– The proxy statement and 2025 Annual Report are available free of charge atwww.nbtbancorp.com/bn/annual-reports.html. Your vote is very important. We urge all stockholders to vote on the matters listed above and described in the proxy statement as soon as possible, whether or not they plan to attend the virtual Annual Meeting.

Proxy Statement

Annual Meeting of Stockholders

May 19, 2026

This proxy statement and accompanying proxy card are furnished to the stockholders of NBT Bancorp Inc. (“NBT” or the “Company”) in connection with the solicitation of proxies on behalf of the board of directors of the Company (the “Board”) to be used at the 2026 Annual Meeting of Stockholders (the “Annual Meeting”). This proxy statement, together with the enclosed proxy card, is being provided to stockholders on or about April 6, 2026.

Annual Meeting Will Be Virtual

NBT will host a virtual Annual Meeting. You may attend the webcast of the meeting via the Internet atwww.virtualshareholdermeeting.com/NBTB2026 when you enter your 16-digit control number included with the Notice of Internet Availability or proxy card. Instructions on how to attend and participate in the Annual Meeting via the webcast are posted onwww.virtualshareholdermeeting.com/NBTB2026. You will be able to vote your shares by following the instructions on the website. You may visitwww.proxyvote.com at any time prior to the Annual Meeting to ask questions of our executive management that may be addressed in the Annual Meeting and to access information about the Company.

The virtual meeting platform is fully supported across browsers (Microsoft Edge, Firefox, Chrome and Safari) and devices (desktops, laptops, tablets and cell phones) running the most updated version of applicable software and plugins. Participants should ensure that they have a strong internet or WiFi connection wherever they intend to participate in the Annual Meeting. Participants should also give themselves plenty of time to log in and ensure that they can hear audio prior to the start of the Annual Meeting.

What Will Be Voted on at the Annual Meeting

At our Annual Meeting, our stockholders will be asked to consider and vote upon the following proposals:

We may take action on the above matters at our Annual Meeting on May 19, 2026, or on any later date to which the Annual Meeting is postponed or adjourned.

We are unaware of other matters to be voted on at our Annual Meeting. If other matters do properly come before our Annual Meeting, including consideration of a motion to adjourn the Annual Meeting to another time and/or place for the purpose of soliciting additional proxies, we intend that the persons named in this proxy will vote the shares represented by the proxies on such matters as determined by a majority of our Board.

Stockholders Entitled to Vote

We have set March 23, 2026 as the record date to determine which of our stockholders will be entitled to vote at our Annual Meeting. Only those stockholders who held their shares of record as of the close of business on that date will be entitled to receive notice of and to vote at our Annual Meeting. As of March 23, 2026, there were 52,009,646 outstanding shares of our common stock held by holders of record. Each of our stockholders on the record date is entitled to one vote per share.

Proxy Statement

Quorum Requirement

The presence of at least a majority of the total number of issued and outstanding shares of common stock entitled to vote at the Annual Meeting is necessary to constitute a quorum at the Annual Meeting. Abstentions and broker non-votes are counted as present for the purpose of determining the presence of a quorum for the transaction of business.

Vote Required to Approve the Proposals

Assuming the presence of a quorum at the Annual Meeting, the voting requirements for the matters presented are as follows:

Our Board urges our stockholders to vote via the Internet, by telephone or by completing, dating and signing the accompanying proxy card and returning it promptly in the enclosed postage-paid envelope.

Effect of Abstentions and Broker Non-Votes

If you abstain from voting on any of the proposals, you will still be considered present at the virtual Annual Meeting for purposes of determining a quorum. Abstentions will have no effect on the outcome of the election of directors. For the purpose of the Say-on-Pay Proposal and the ratification of the appointment of our independent registered public accounting firm, abstentions will have the effect of a vote against the proposal.

Broker non-votes are proxies received from brokers or other nominees holding shares on behalf of their clients who have not been given specific voting instructions from their clients with respect to non-routine matters. Brokers who hold their customers’ shares in “street name” may, under the applicable rules of the exchange and other self-regulatory organizations of which the brokers are members, sign and submit proxies for such shares and may vote such shares on “routine” matters. Proposals 1 and 2 are considered “non-routine” and Proposal 3 is considered “routine” under the applicable rules. Broker non-votes will not be counted as a vote cast or entitled to vote on any matter presented at the Annual Meeting and will therefore have no effect on the outcome of the voting on any matter presented at the Annual Meeting.

Voting Your Shares

Our Board is soliciting proxies from our stockholders. When you deliver a valid proxy, the shares represented by that proxy will be voted by a named agent in accordance with your instructions.

If you are a stockholder and vote by proxy but make no specification on your proxy card that you have otherwise properly executed, the named persons will vote the shares represented by your proxy:

If any other matters are properly brought before the Annual Meeting, the persons named in the proxy will vote the shares represented by such proxy or such matters as determined by a majority of the board of directors.

If you are a stockholder whose shares are registered in your name, you may vote your shares by using one of the following methods:

Via the Internet. If you hold NBT common stock in your own name and not through a broker or other nominee, you can vote your shares of NBT common stock electronically via the Internet. Internet voting procedures are designed to authenticate stockholders by using the individual control number on your proxy card. If you vote via the Internet, you do not need to return your proxy card.

Proxy Statement

Mail. To grant your proxy by mail, please complete your proxy card and sign, date and return it in the enclosed envelope. To be valid, a returned proxy card must be signed and dated.

Telephone. If you hold NBT common stock in your own name and not through a broker or other nominee, you can vote your shares of NBT common stock by telephone by dialing the toll-free telephone number 1-800-690-6903. Telephone voting is available 24 hours a day until 11:59 p.m. Eastern Time on May 18, 2026 for shares held directly and by 11:59 p.m. Eastern Time on May 14, 2026 for shares held in a 401(k) or other employee benefit plan that allows participants to invest and hold NBT common stock. Telephone voting procedures are designed to authenticate stockholders by using the individual control number on your proxy card. If you vote by telephone, you do not need to return your proxy card.

If your shares are registered in the name of a bank or brokerage firm you will receive instructions from your holder of record that must be followed in order for the record holder to vote the shares per your instructions. Many banks and brokerage firms have a process for their beneficial holders to provide instructions over the telephone or via the Internet.

Changing Your Vote

Any NBT stockholder of record giving a proxy may revoke the proxy in one or more of the following ways:

You must send any written notice of revocation or subsequent proxy to NBT Bancorp Inc., 52 South Broad Street, Norwich, New York 13815, Attention: M. Randolph Sparks, Corporate Secretary. Any later-dated proxy submitted by telephone or via the Internet must be submitted prior to 11:59 p.m. Eastern Time on May 18, 2026 for shares held directly and by 11:59 p.m. Eastern Time on May 14, 2026 for shares held in a 401(k) or other employee benefit plan that allows participants to invest and hold NBT common stock.

If you hold shares through a bank or brokerage firm, you must contact that firm to revoke any prior voting instructions.

Solicitation of Proxies and Costs

We will bear our own costs of soliciting proxies. We will reimburse brokerage houses, fiduciaries, nominees and others for their out-of-pocket expenses in forwarding proxy materials to owners of shares of our common stock held in their names. In addition to the solicitation of proxies by use of the mail, we may solicit proxies from our stockholders by directors, officers and employees acting on our behalf in person or by telephone, facsimile or other appropriate means of communications. We will not pay any additional compensation, except for reimbursement of reasonable out-of-pocket expenses, to our directors, officers and employees in connection with the solicitation. You may direct any questions or requests for assistance regarding this proxy statement to M. Randolph Sparks, Corporate Secretary, by telephone at (607) 337-6141 or by email atrsparks@nbtbank.com.

Annual Report

A copy of our 2025 Annual Report accompanies this proxy statement. NBT is required to file an annual report on Form 10-K for its 2025 fiscal year with the U.S. Securities and Exchange Commission (“SEC”). Stockholders may obtain, free of charge, a copy of the Form 10-K by writing to: NBT Bancorp Inc., 52 South Broad Street, Norwich, New York 13815, Attention: M. Randolph Sparks, Corporate Secretary. Our annual report on Form 10-K is available on our websitewww.nbtbancorp.com.

REGARDLESS OF THE NUMBER OF SHARES YOU OWN, YOUR VOTE IS IMPORTANT TO US. PLEASE VOTE BY TELEPHONE, VIA THE INTERNET USING THE INTERNET ADDRESS ON YOUR PROXY CARD, OR BY COMPLETING, SIGNING, DATING AND PROMPTLY RETURNING THE ACCOMPANYING PROXY CARD IN THE ENCLOSED POSTAGE-PAID ENVELOPE.

Certain Terms

Certain terms that we use in the accompanying proxy statement have particular meanings, as set forth below.

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