The law requires that registered sex offenders provide police with a complete list of their user names, screen names, email addresses and Internet service providers.
Violators of the law face a possible three years in prison.
Courthouse News Service said after the law passed in November, two sex offenders challenged it, arguing it impeded free speech.
U.S. District Judge Thelton Henderson issued a preliminary injunction Friday and said the interest in protecting the public from sex offenders and human trafficking does not outweigh privacy interests.
Henderson noted only 1 percent of arrests for sex crimes are facilitated by technology, Courthouse News Service reported.
"Plaintiffs enjoy no lesser right to anonymous speech simply because they are unpopular," Henderson wrote.