Exhibit 5.1

Hogan Lovells US LLP

Columbia Square

555 Thirteenth Street, NW

Washington, DC 20004

T +1 202 637 5600

F +1 202 637 5910

www.hoganlovells.com

October 24, 2014

Board of Directors

Radius Bancorp Inc.

One Harbor Street, Suite 201

Boston, Massachusetts 02210

Ladies and Gentlemen:

We are acting as counsel to Radius Bancorp Inc., a Delaware corporation (the “Company”), in connection with its registration statement on Form S-1, as amended, (the “Registration Statement”), filed with the Securities and Exchange Commission under the Securities Act of 1933, as amended (the “Act”), relating to the proposed public offering of up to 5,750,000 shares of common stock, par value $0.0001 per share, of the Company (the “Common Stock”), including (a) 2,080,551 shares of Common Stock to be issued and sold by the Company (the “Company Shares”) and (b) 3,669,449 shares of outstanding Common Stock to be resold by New England Carpenters’ Pension Fund, New England Carpenters’ Guaranteed Annuity Fund and Empire State Carpenters’ Pension Fund as selling shareholders (the “Selling Shareholders,” and such shares, the “Selling Shareholder Shares”). This opinion letter is furnished to you at your request to enable you to fulfill the requirements of Item 601(b)(5) of Regulation S-K, 17 C.F.R. § 229.601(b)(5), in connection with the Registration Statement.

For purposes of this opinion letter, we have examined copies of such agreements, instruments and documents as we have deemed an appropriate basis on which to render the opinions hereinafter expressed. In our examination of the aforesaid documents, we have assumed the genuineness of all signatures, the legal capacity of all natural persons, the accuracy and completeness of all documents submitted to us, the authenticity of all original documents, and the conformity to authentic original documents of all documents submitted to us as copies (including telecopies). As to all matters of fact, we have relied on the representations and statements of fact made in the documents so reviewed, and we have not independently established the facts so relied on. This opinion letter is given, and all statements herein are made, in the context of the foregoing.

This opinion letter is based as to matters of law solely on the Delaware General Corporation Law, as amended. We express no opinion herein as to any other laws, statutes, ordinances, rules or regulations. As used herein, the term “Delaware General Corporation Law, as amended” includes the statutory provisions contained therein, all applicable provisions of the Delaware Constitution and reported judicial decisions interpreting these laws.

Based upon, subject to and limited by the foregoing, we are of the opinion that following (i) execution and delivery by the Company of the Underwriting Agreement, (ii) effectiveness of the Registration Statement, (iii) issuance of the Company Shares pursuant to the terms of the Underwriting Agreement, (iv) the sale of the Selling Shareholder Shares pursuant to the Underwriting Agreement, and (v) receipt by the Company of the consideration for the Shares specified in the resolutions of the Board of Directors and the Pricing Committee of the Board of Directors, the Company Shares and the Selling Shareholder Shares will be validly issued, fully paid and nonassessable.

Hogan Lovells US LLP is a limited liability partnership registered in the District of Columbia. “Hogan Lovells” is an international legal practice that includes Hogan Lovells US LLP and Hogan Lovells International LLP, with offices in: : Alicante Amsterdam Baltimore Beijing Brussels Caracas Colorado Springs Denver Dubai Dusseldorf Frankfurt Hamburg Hanoi Ho Chi Minh City Hong Kong Houston Johannesburg London Los Angeles Luxembourg Madrid Mexico City Miami Milan Monterrey Moscow Munich New York Northern Virginia Paris Philadelphia Rio de Janeiro Rome San Francisco São Paulo Shanghai Silicon Valley Singapore Tokyo Ulaanbaatar Warsaw Washington DC Associated offices: Budapest Jakarta Jeddah Riyadh Zagreb. For more information see www.hoganlovells.com

Board of Directors

Radius Bancorp Inc.

- 2 -

October 24, 2014

This opinion letter has been prepared for use in connection with the Registration Statement. We assume no obligation to advise you of any changes in the foregoing subsequent to the effective date of the Registration Statement.

We hereby consent to the filing of this opinion letter as Exhibit 5.1 to the Registration Statement and to the reference to this firm under the caption “Legal Matters” in the prospectus constituting a part of the Registration Statement. In giving this consent, we do not thereby admit that we are an “expert” within the meaning of the Act.

Very truly yours,

/s/ Hogan Lovells US LLP

HOGAN LOVELLS US LLP

Radius Bancorp Inc. S-1/A