The Supreme Court ruled Wednesday that police must obtain warrants before snooping through people’s cellphones, delivering a unanimous decision that begins to update legal understanding of privacy rules to accommodate 21st-century technology.

Police agencies argued that searching through data on cellphones was no different from asking someone to turn out his pockets, but the justices rejected that, saying a cellphone holds the most personal and intimate details of someone’s life and falls squarely within the Fourth Amendment’s privacy protections.

“The fact that technology now allows an individual to carry such information in his hand does not make the information any less worthy of the protection for which the Founders fought,” Chief Justice John G. Roberts Jr. wrote in the unanimous opinion. “Our answer to the question of what police must do before searching a cell phone seized incident to an arrest is accordingly simple — get a warrant.”

The justices even said police cannot check a cellphone’s call log because it could contain more information than phone numbers, and perusing the call log is a violation of privacy that can be justified only with a court-issued warrant.

Legal analysts said the ruling will change the way police operate but predicted investigators will adjust.

Privacy advocates, meanwhile, said the ruling should ignite a broader rethinking of protections at a time when Americans are putting more personal information online.

Sen. Patrick J. Leahy, Vermont Democrat and chairman of the Senate Judiciary Committee, said Congress should extend the same privacy protections to email. Sen. Ron Wyden, Oregon Democrat and a member of the Senate Select Committee on Intelligence, said GPS information should be protected.

“At a time when the details of a person’s whole life can be in their pocket, the Supreme Court sent a clear message that Fourth Amendment rights still apply in the digital era,” Mr. Wyden said.

Congress is moving to rein in the National Security Agency’s phone-records snooping program, and the court ruling Wednesday is likely to boost those efforts.

In his ruling, Chief Justice Roberts said cellphones can hold the equivalent of millions of pages of documents that are qualitatively different from what someone otherwise might carry.

He said a search of the data could lay bare someone’s entire personal history, including medical records and “specific movements down to the minute.”

The chief justice cited court precedent that found a difference between asking someone to turn out his pockets versus “ransacking his house for everything which may incriminate him” — and the court found that a cellphone falls into that second category.

Complicating matters further is the question of where the information is stored. The Obama administration and the state of California, both of which sought to justify cellphone searches, acknowledged that remotely stored data couldn’t be searched. Chief Justice Roberts said with cloud computing, it’s sometimes impossible to know the difference.

The court did carve out exceptions for “exigencies” such as major security threats.

Wednesday’s ruling combines two separate cases argued this term before the court.

In one of those cases, David Riley was convicted of a shooting after police in California stopped him for a traffic violation, found weapons in his car and began poking through his cellphone. They found photos on the phone that connected him with the shooting.

In the second case, Brima Wurie was arrested after police said they saw him take part in a drug buy. They saw his phone receive a number of text messages, accessed the call log and traced the number back to an apartment. They obtained a search warrant and found weapons and drugs in the apartment.

During oral argument, the Obama administration warned of a potential technological “arms race” between police labs on one hand and phone manufacturers and criminals on the other.

Government attorneys said encryption technology makes it difficult for police to wait for a warrant.

But Chief Justice Roberts said police can take steps to help investigators, such as holding on to the cellphone while they wait for a court to issue a warrant.

“We cannot deny that our decision today will have an impact on the ability of law enforcement to combat crime,” the chief justice wrote, but concluded: “Privacy comes at a cost.”

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