The justices said that because cellphones potentially contain vast amounts of personal information unrelated to a person's arrest, they should be off limits to a police search without a warrant.
"Modern cellphones are not just another technological convenience. With all they contain and all they may reveal, they hold for many Americans the privacies of life," Chief Justice John Roberts said in writing for the court.
How they voted
In the majority
Stephen Breyer John Roberts, Anthony Kennedy, Ruth Bader Ginsburg, Sonia Sotomayor, Elena Kagan, Antonin Scalia, Clarence Thomas, Samuel Alito
"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."
The case pitted pitted privacy advocates and defense lawyers against law enforcement and the federal government, who argued that cellphone searches immediately after an arrest are vital to ensure evidence isn't tampered with or destroyed.
While the Fourth Amendment says police generally need a warrant before they can conduct a search, the Supreme Court in recent decades has made exceptions for belongings people carry when arrested.
The Justice Department argued that police are interested only in evidence pertinent to a person's arrest and aren't interested in gleaning personal information from cellphones.
The government also said that searches of cellphones pose little legal difference than when police search the contents of a person's wallet at the time of an arrest, which they already have the authority to do.
But Roberts said the solution for police was “accordingly simple: Get a warrant.”
The justices also suggested it was unrealistic for police to immediately know what information is relevant and what isn't, particularly in the heat of making an arrest.
And with 90 percent of adult Americans owning a cellphone, privacy rights advocates worried that police could trump up dubious arrest charges on suspects to get access to their phones.
The ruling involved a pair of similar cases in San Diego and Boston.
In the California case, a state court upheld the conviction of David Leon Riley after police found evidence on his smartphone that he belonged to a gang and was involved in a gang-related shooting. Video and photographs found on the smartphone helped persuade a jury to convict Riley of attempted murder and other charges.
In the other case, a federal appeals court in Massachusetts said the warrantless search of a cellphone belonging to Brima Wurie violated the Fourth Amendment. After arresting him on suspicion of selling crack cocaine, police examined the call log on his older model "flip phone" and used the information to determine where he lived. When they searched his home after obtaining a warrant, they found crack, marijuana, a gun and ammunition.