Exhibit 5.1

Reed Smith LLP

599 Lexington Avenue

New York, NY 10022-7650

+1 212 521 5400

Fax +1 212 521 5450

reedsmith.com

September 1, 2016

Juniper Pharmaceuticals, Inc.

33 Arch Street

Suite 3110

Boston, Massachusetts 02110

Re: Registration Statement on Form S-8

Ladies and Gentlemen:

We have acted as counsel for Juniper Pharmaceuticals, Inc., a Delaware corporation (the “Company”), in connection with the registration under the Securities Act of 1933, as amended (the “Securities Act”), on the Form S-8 (the “Registration Statement”) filed by the Company on September 1, 2016 with the Securities and Exchange Commission (the “Commission”) of (i) up to 2,482,299 shares of common stock, par value $0.01 per share (the “Common Stock”), of the Company issuable under the Company’s Amended and Restated 2015 Long Term Incentive Plan (the “2015 Amended and Restated Plan”), and (ii) 225,000 shares of Common Stock issuable to Alicia Secor upon exercise of the inducement nonqualified stock option granted to her on July 20, 2016 (the “Inducement Option,” and, together with the 2015 Amended and Restated Plan, the “Plans”). The shares being registered pursuant to the Registration Statement are collectively referred to herein as the “Shares”.

This opinion is being furnished in connection with the requirements of Item 601(b)(5) of Regulation S-K under the Securities Act, and no opinion is expressed herein as to any matter pertaining to the contents of the Registration Statement or the prospectus forming a part thereof, other than as to the issue of the Shares.

In connection with the foregoing, we have examined originals or copies, certified or otherwise identified to our satisfaction, of such documents, corporate records, certificates of public officials, and other instruments as we have deemed necessary or advisable for the purpose of rendering this opinion. As to questions of fact material to this opinion that have not been independently established, we have relied upon certificates or comparable documents of officers and representatives of the Company.

Based upon the foregoing, we are of the opinion that, as of the date hereof, when the Shares shall have been duly registered on the books of the transfer agent and registrar therefor in the name or on behalf of the recipients, and have been issued by the Company against payment therefor in the circumstances contemplated by the Plans, and assuming in each case that the individual issuances, grants or awards under the Plans are duly authorized by all necessary corporate action of the Company and duly issued, granted or awarded and exercised in accordance with the requirements of law and the Plans (and the agreements and awards duly adopted thereunder and in accordance therewith), the issue and sale of the Shares will have been duly authorized by all necessary corporate action of the Company and the Shares will be validly issued, fully paid and non-assessable.

ABU DHABI • ATHENS • BEIJING • CENTURY CITY • CHICAGO • DUBAI • FRANKFURT • HONG KONG • HOUSTON • KAZAKHSTAN • LONDON • LOS ANGELES • MUNICH • NEW YORK • NORTHERN VIRGINIA PARIS • PHILADELPHIA • PITTSBURGH • PRINCETON •RICHMOND • SAN FRANCISCO • SHANGHAI • SILICON VALLEY • SINGAPORE • WASHINGTON, D.C. • WILMINGTON

Juniper Pharmaceuticals, Inc.

September 1, 2016

Page 2

In giving the opinions in this letter, we have assumed that at the time of the issuance of the Shares, (i) the Company will validly exist and be duly qualified and in good standing under the laws of its jurisdiction of incorporation, (ii) the Company will have the necessary corporate power and due authorization, (iii) the terms of the Shares and of their issuance and sale will have been established in conformity with and so as not to violate, or result in a default under or breach of, the certificate of incorporation and bylaws of the Company and any applicable law or any agreement or instrument binding upon the Company and so as to comply with any requirement or restriction imposed by any court or governmental or regulatory body having jurisdiction over the Company, (iv) sufficient shares of Common Stock will be authorized for issuance under the certificate of incorporation of the Company that have not otherwise been issued or reserved or committed for issuance, (v) the consideration paid by the recipients upon issuance of each Share will not be less than the par value thereof, and (vi) none of the documents reviewed by us as of the date hereof have been rescinded or modified in any manner inconsistent with the opinion expressed herein.

Our opinion is based (i) as to matters of law solely on applicable provisions of the laws of the state of Delaware, and we express no opinions as to any other laws, statutes, ordinances rules, or regulations and (ii) on such corporate records as we deemed necessary or appropriate.

We consent to the filing of this opinion as an exhibit to the Registration Statement. In giving this consent, we do not thereby admit that we are within the category of persons whose consent is required under Section 7 of the Act or the rules and regulations of the Commission.

Very truly yours,

/s/ Reed Smith LLP

REED SMITH LLP

a Limited Liability Partnership

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