California will require police to get a court order before they can search messages, photos and other digital data stored on phones or company servers.
Civil-liberties advocates are calling the new law that takes effect on January 1 an important advance and highlights the need for nationwide protections.
The California Electronic Communications Privacy Act was signed by Governor Jerry Brown on Thursday. It's only the third of its kind in the US.
While some states guarantee some of its protections, only Maine and Utah previously had comprehensive laws on the books, noted Hanni Fakhoury, senior staff attorney for the Electronic Frontier Foundation.
"It's an expansive bill and this being California, it covers a lot of people," Fakhoury said of the state with a population of about 39 million.
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"It's an important thing and a good development."
The digital rights group, along with the American Civil Liberties Union, news organisations and tech companies, worked for the bill's passage.
They argued previous California law dating back to the 1980s was in desperate need of an update given the dramatic changes in the digital world.
But the bill's opponents, including several California police groups, argue the measure will hamper the ability of law enforcement to investigate child pornographers and others who commit crimes online.
Law-enforcement requests for people's electronic information, particularly from technology companies such as Google and Twitter, have skyrocketed in recent years, said Nicole Ozer, technology and civil liberties policy director for the ACLU of California.
Previously, all that was generally needed to get the information was a subpoena. Now, under the new law, a warrant will be required in most cases.
