Qualys (QLYS) S-8Registration of securities for employees

Exhibit 5.1

February 21, 2020

Qualys, Inc.

919 East Hillsdale Boulevard, 4^th Floor

Foster City, California 94404

Re: Registration Statement on FormS-8

Ladies and Gentlemen:

We have examined the Registration Statement on FormS-8 (the “Registration Statement”) to be filed by Qualys, Inc., a Delaware corporation, with the Securities and Exchange Commission on or about the date hereof, in connection with the registration under the Securities Act of 1933, as amended, of 1,957,314 additional shares of common stock, par value $0.001 per share (the “Shares”) to be issued under the 2012 Equity Incentive Plan (the “Plan”). As your legal counsel, we have reviewed the actions proposed to be taken by you in connection with the issuance and sale of the Shares to be issued under the Plan.

It is our opinion that the Shares, when issued and sold in the manner referred to in the Plan and pursuant to the agreements that accompany the Plan, will be legally and validly issued, fully paid and nonassessable.

We consent to the use of this opinion as an exhibit to the Registration Statement, and further consent to the use of our name wherever appearing in the Registration Statement and any amendments thereto.