U.S. District Judge Thelton Henderson's decision keeps in place an existing injunction against a section of Proposition 35 that requires registered sex offenders to give authorities a list of their Internet providers and screen names.
The ruling does not affect other portions of the November ballot initiative, which toughened penalties on those convicted of human trafficking.
The San Francisco-based judge ruled that opponents represented by the American Civil Liberties Union of Northern California and the Electronic Frontier Foundation are likely to prevail in their argument that the provision violates the First Amendment.
Had Henderson not acted, more than 73,000 registered sex offenders would have had to provide their online identities to law enforcement. The initiative also would have required them to report any new account or screen name within 24 hours.
The initiative passed with 81 percent support. However, Henderson ruled that the state attorney general was not able to show that the reporting requirements were narrowly crafted so they would solely be used to fight online sex offenses.
"The government has a legitimate interest in protecting individuals from online sex offenses and human trafficking," Henderson wrote in his 21-page decision. But he concluded that sex offenders "enjoy no lesser right to anonymous speech simply because they are 'unpopular.'"
The injunction will stand until the state proves otherwise.
Chris Kelly of the Safer California Foundation said in a statement that he is confident the provision will eventually be upheld by the courts, as similar measures have prevailed in other states.