In an opinion written by Chief Justice John Roberts, the court said that the right of police to search an arrested suspect at the scene without a warrant does not extend in most circumstances to data held on a cellphone. There are some emergency situations in which a warrantless search would be permitted, the court noted.
The unanimous 9-0 ruling goes against law enforcement agencies, including the U.S. Department of Justice, which wanted more latitude to search without having to obtain a warrant. The ruling is a win for digital privacy advocates.
"We cannot deny that our decision today will have an impact on the ability of law enforcement to combat crime," Roberts wrote.
The right to privacy, he said, "comes at a cost."
The court was weighing two separate cases that pitted evolving expectations of privacy against the interests of the law enforcement community as the court for the first time weighed the increasingly ubiquitous role of cellphones in modern day life.
The ruling appears to be in line with public opinion based on results of a Reuters/Ipsos opinion poll, which found that 60.7 percent of people surveyed said police should not be allowed to search cellphones without a warrant.
Cellphones, initially used purely to make calls, now contain a wealth of personal information about the owner, including photographs, video and social media content. According to a 2013 report by the Pew Research Center, 91 percent of adult Americans have a cell phone, more than a half of them smartphones that can connect to the Internet.
Concerns about increasing government encroachment on personal privacy, especially in relation to electronic communications, has surged into the public eye over the last year after disclosures made by former National Security Agency contractor Edward Snowden about government surveillance.
Roberts acknowledged the unique nature of cell phones in contemporary life, noting that "the proverbial visitor from Mars might conclude they were an important feature of human anatomy."
Justice Samuel Alito, who is a former federal prosecutor, wrote a separate opinion agreeing with the outcome but saying that Congress and state legislatures should be allowed to pass laws that would narrow the scope of the ruling once they have considered "the legitimate needs of law enforcement."
The defendants challenging their convictions, David Riley and Brima Wurie, said evidence found on their phones should not have been used at trial because the searches were conducted without court-issued warrants.
The circumstances in the two cases, one from Massachusetts and one from California, were different, both in terms of the scope of the search and the type of cellphone used. Wurie had a basic flip phone while Riley had a more sophisticated smartphone.
The court decided the two cases together, finding that both searches were unconstitutional.
The legal question was whether the Fourth Amendment to the U.S. Constitution, which bars unreasonable searches, requires police following an arrest to get court approval before a cellphone can be searched.
Riley was convicted of three charges relating to an August 2009 incident in San Diego in which shots were fired at an occupied vehicle. Local prosecutors linked him to the crime in part based on a photograph police found on his smartphone
In the other case, police searched Wurie's cellphone without a warrant after his September 2007 arrest for suspected drug dealing. Officers used the device, which was not a smartphone, to find a phone number that took them to Wurie's house in Boston, where drugs, a gun and cash were found.
The cases are Riley v. California, 13-132 and U.S. v. Wurie, 13-212.