The ruling came in a Washington drug dealer investigation. A U.S. appeals court in the nation's capital said the data obtained by the GPS device was obtained without a warrant as required by the Fourth Amendment, and suppressed it.
The appeals court said other evidence obtained without using the GPS device was admissible because the suspect had no "reasonable expectation of privacy" for a vehicle on the public streets.
The Supreme Court agreed with the appeals court.
Writing for the majority, Justice Antonin Scalia said the government's attachment of "the GPS device to the vehicle, and its use of that device to monitor the vehicle's movements, constitutes a search under the Fourth Amendment."
The amendment bans "unreasonable searches and seizures." Scalia said the installation of the GPS device was consistent with the meaning of a search when the Fourth Amendment was adopted.
Four other justices signed on to Scalia's opinion. Four others agreed with the judgment in concurring opinions, but for different reasons.
In 2004, a joint Safe Streets Task Force of the FBI and Washington police focused on Antoine Jones, who owned and operated a nightclub in the District of Columbia and was suspected of narcotics violations.
Agents obtained a warrant from a federal judge authorizing them to secretly install and monitor a GPS tracking device on Jones' Jeep Grand Cherokee -- as long as it was installed within 10 days and only within the District of Columbia.
But the agents did not install the device until 11 days after the warrant was issued, and while the Jeep was parked in a public parking lot in Maryland, not the district.
In October 2005, agents executed search warrants at various locations, recovering large amounts of cash, cocaine, drug paraphernalia and firearms and Jones was indicted.
Eventually, Jones was convicted of a drug count and sentenced to life in prison and the forfeiture of $1 million in proceeds from drug trafficking.
But the U.S. Court of Appeals for the District of Columbia reversed the conviction.