Exhibit 5

LUSE GORMAN, PC

ATTORNEYS AT LAW

5335 Wisconsin Avenue, NW, Suite 780

Washington, D.C. 20015

Telephone (202) 274-2000

Facsimile (202) 362-2902

www.luselaw.com

WRITER’S DIRECT DIAL NUMBER

(202) 274-2000

December 12, 2016

Board of Directors

PCSB Financial Corporation

2651 Strang Blvd., Suite 100

Yorktown Heights, New York 10595

Members of the Board:

You have requested the opinion of this firm as to certain matters in connection with the offer and sale, and contribution to PCSB Community Foundation (the “Charitable Foundation”), of the shares of common stock, par value $0.01 per share (the “Common Stock”), of PCSB Financial Corporation (the “Company”). We have reviewed the Company’s Articles of Incorporation and its Registration Statement on Form S-1 (the “Form S-1”), the Plan of Conversion of PCSB Bank (the “Plan”), as well as applicable statutes and regulations governing the Company and the offer and sale of the Common Stock. The opinion expressed below is limited to the laws of the State of Maryland (which includes applicable provisions of the Maryland General Corporation Law, the Maryland Constitution and reported judicial decisions interpreting the Maryland General Corporation Law and the Maryland Constitution).

We are of the opinion that upon the declaration of effectiveness of the Form S-1, the Common Stock, when issued and sold, and, in the case of the Charitable Foundation, contributed in accordance with the Plan, will be legally issued, fully paid and non-assessable.

We hereby consent to our firm being referenced under the caption “Legal Matters” and to the filing of this opinion as an exhibit to the Form S-1.