SAN FRANCISCO—A state appellate court in California has revived a defamation lawsuit that software company ZL Technologies Inc. filed against the authors of critical posts on the anonymous job review website Glassdoor.com.

The ruling overturns a finding from the trial judge in Marin County Superior Court who declined to force Glassdoor Inc. to hand over the names of the anonymous posters after finding the reviews contained opinion statements protected under the First Amendment.