Justice Clarence Thomas, joined by the court's four liberals, wrote the majority opinion.
In 2013, Allen Alleyne was charged in Richmond, Va., with using a firearm in the commission of a crime. A state judge ruled that his accomplice brandished a gun during the commission of a robbery.
Federal law carries a 5-year mandatory minimum sentence, and a provision increases that to a 7-year minimum "if the firearm is brandished," while another increases that to a 10-year minimum "if the firearm is discharged."
A jury found Alleyne guilty, but said he did not brandish a firearm. However, a pre-sentence report recommended a 7-year term.
Alleyne's lawyers argued the jury did not find a firearm had been brandished by the defendant, and the extended sentence violated his right to a jury trial under the Sixth Amendment.
The federal judge overruled the objection, citing Supreme Court precedent that judicial fact-finding that increases a sentence is constitutional, and an appeals court agreed.
But Thomas' opinion threw out the lower courts' rulings, and sent the case back down for a rehearing.
"Because mandatory minimum sentences increase the penalty for a crime, any fact that increases the mandatory minimum is an 'element' that must be submitted to the jury," Thomas said.