Exhibit 5.1

Darren K. DeStefano

T: 703 456 8034

ddestefano@cooley.com

April 11, 2019

PhaseBio Pharmaceuticals, Inc.

1 Great Valley Parkway

Suite 30

Malvern, Pennsylvania 19355

Ladies and Gentlemen:

We have acted as counsel to PhaseBio Pharmaceuticals, Inc., a Delaware corporation (the “Company”), in connection with the filing by the Company of a Registration Statement on Form S-1 (the “Registration Statement”) with the Securities and Exchange Commission, pursuant to Rule 462(b) promulgated under the Securities Act of 1933, as amended, covering an underwritten public offering of up to 674,475 shares of the Company’s common stock (the “Shares”), par value $0.001 per share, including up to 87,975 Shares that may be sold pursuant to the exercise of an over-allotment option to be granted to the underwriters. The Registration Statement incorporates by reference the registration statement on Form S-1 (No. 333-230783), which was declared effective on April 11, 2019 (the “Prior Registration Statement”), including the prospectus which forms a part of the Prior Registration Statement (the “Prospectus”).

In connection with this opinion, we have examined and relied upon (a) the Registration Statement, the Prior Registration Statement and the Prospectus, (b) the Company’s Amended and Restated Certificate of Incorporation and Amended and Restated Bylaws, each as amended and in effect as of the date hereof and (c) originals or copies certified to our satisfaction of such records, documents, certificates, memoranda and other instruments as in our judgment are necessary or appropriate to enable us to render the opinion expressed below.

We have assumed the genuineness and authenticity of all documents submitted to us as originals and the conformity to originals of all documents submitted to us as copies and the due execution and delivery, other than by the Company, of all documents where due execution and delivery are a prerequisite to the effectiveness thereof. As to certain factual matters, we have relied upon a certificate of an officer of the Company and have not sought independently to verify such matters.

Our opinion is expressed only with respect to the General Corporation Law of the State of Delaware. We express no opinion to the extent that any other laws are applicable to the subject matter hereof and no opinion and provide no assurance as to compliance with any federal or state securities law, rule or regulation.

On the basis of the foregoing, and in reliance thereon, we are of the opinion that the Shares, when sold and issued against payment therefor as described in the Registration Statement and the Prospectus, will be validly issued, fully paid and non-assessable.

We consent to the reference to our firm under the caption “Legal Matters” in the Prospectus included in the Registration Statement and to the filing of this opinion as an exhibit to the Registration Statement.

Sincerely,

Cooley LLP
By:	/s/ Darren K. DeStefano
	Darren K. DeStefano

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