EX-5.1

Exhibit 5.1

Simpson Thacher & Bartlett LLP

425 LEXINGTON AVENUE

NEW YORK, NY 10017-3954

TELEPHONE: +1-212-455-2000

FACSIMILE: +1-212-455-2502

Direct Dial Number

E-mail Address

August 12, 2019

People’s United Financial, Inc.

850 Main Street

Bridgeport, Connecticut 06604

Ladies and Gentlemen:

We have acted as counsel to People’s United Financial, Inc., a Delaware corporation (the “Company”), in connection with the Registration Statement on Form S-4 (the “Registration Statement”) filed by the Company with the Securities and Exchange Commission (the “Commission”) under the Securities Act of 1933, as amended, relating to the issuance by the Company of up to 46,693,993 shares of Common Stock, par value $0.01 per share (the “Shares”), in connection with the merger contemplated by the Agreement and Plan of Merger, dated as of July 15, 2019 (the “Merger Agreement”), by and between United Financial Bancorp, Inc., a Connecticut corporation, and the Company.

We have examined the Registration Statement, the Merger Agreement and a form of the share certificate, which has been filed with the Commission as an exhibit to the Registration Statement. In addition, we have examined, and have relied as to matters of fact upon, originals, or duplicates or certified or conformed copies, of such records, agreements, documents and other instruments and such certificates or comparable documents of public officials and of officers and representatives of the Company and have made such other investigations as we have deemed relevant and necessary in connection with the opinions hereinafter set forth.

In rendering the opinion set forth below, we have assumed the genuineness of all signatures, the legal capacity of natural persons, the authenticity of all documents submitted to us as originals, the conformity to original documents of all documents submitted to us as duplicates or certified or conformed copies and the authenticity of the originals of such latter documents.

Based upon the foregoing, and subject to the qualifications, assumptions and limitations stated herein, we are of the opinion that when the Shares have been issued in accordance with the terms and conditions set forth in the Merger Agreement, the Shares will be validly issued, fully paid and nonassessable.

We do not express any opinion herein concerning any law other than the Delaware General Corporation Law.

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