By a 9-0 vote, the justices said smart phones and other electronic devices were not in the same category as wallets, briefcases, and vehicles -- all currently subject to limited initial examination by law enforcement.
Generally such searches are permitted if there is "probable cause" that a crime has been committed, to ensure officers' safety and prevent destruction of evidence.
Criminal suspects in Massachusetts and California were separately convicted, in part, after phone numbers, text messages, photos and addresses obtained from personal electronic devices linked them to drug and gang activity.
Those cases were appealed to the high court, giving it an opportunity to re-enter the public debate over the limits of Americans' privacy rights, with a focus on the ubiquitous cellphone and its vast storage of information and video.
These appeals were not related to the recent mass surveillance of phone metadata by the National Security Agency, which has raised similar constitutional concerns.
"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 ruling said. "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."