The majority said the state law "does not comport with the First Amendment."
The state law, with its fine of as much as $1,000 for each violation, was challenged by groups representing the video-game and software industries. A federal judge said the law violated the First Amendment, and issued an injunction against its implementation. A federal appeals court agreed.
A majority of the U.S. Supreme Court agreed with the lower courts. An opinion filed by conservative Justice Antonin Scalia said: "Video games qualify for First Amendment protection. Like protected books, plays, and movies, they communicate ideas through familiar literary devices and features distinctive to the medium."
Citing Supreme Court precedent, the opinion said "the basic principles of freedom of speech ... do not vary" with a new and different communication medium."
It added: "This country has no tradition of specially restricting children's access to depictions of violence. And California's claim that 'interactive' video games present special problems, in that the player participates in the violent action on screen and determines its outcome, is unpersuasive."
"Because the act imposes a restriction on the content of protected speech, it is invalid unless California can demonstrate that it passes strict scrutiny -- that is, unless it is justified by a compelling government interest and is narrowly drawn to serve that interest," Scalia said.
"California cannot meet that standard," he added later. "At the outset, it acknowledges that it cannot show a direct causal link between violent video games and harm to minors. Rather ... the state claims that it need not produce such proof because the Legislature can make a predictive judgment that such a link exists, based on competing psychological studies."
One of the two dissenters, Justice Stephen Breyer, said: "This case is ultimately less about censorship than it is about education. Our Constitution cannot succeed in securing the liberties it seeks to protect unless we can raise future generations committed cooperatively to making our system of government work. Education, however, is about choices. Sometimes, children need to learn by making choices for themselves. Other times, choices are made for children -- by their parents, by their teachers and by the people acting democratically through their governments. In my view, the First Amendment does not disable government from helping parents make such a choice here -- a choice not to have their children buy extremely violent, interactive video games, which they more than reasonably fear pose only the risk of harm to those children."