The 7-2 ruling comes in a case out of Arizona, one of nine states where judges could determine whether a crime was heinous enough to deserve the death penalty.
It was unclear Monday how many death row inmates would have to be resentenced by juries in those states because of the Supreme Court decision.
"It has been said that most legal controversies turn on two questions: 'Who decides?', and 'Where do you draw the line?'," Justice Ruth Bader Ginsburg said from the bench Monday. "This case presents a question of 'Who decides: judge or jury?' The context is capital murder. The issue, life or death."
Speaking for the court majority, Ginsburg pointed out that two Supreme Court precedents were apparently at war with each other.
In 1990's Walton vs. Arizona, the Supreme Court upheld the state's method of allowing a judge to determine whether the elements of a murder meant a convicted defendant should be sentenced to death.
However, the Supreme Court decided two years ago in Apprendi vs. New Jersey that the Sixth Amendment forbids "exposing a defendant to a prison term longer than the maximum sentence he could receive based solely on the facts resolved by the jury's verdict," Ginsburg said.
In others words, once a defendant was convicted of one thing, say drug trafficking, a prosecutor could not bring up another unproved violation to lengthen the sentence for drug trafficking.
Any fact that lengthened a sentence, and was not part of the original crime, must be ruled on by a jury and proved beyond a reasonable doubt, just like the original crime.
The case that brought Monday's decision began in November 1994 in Maricopa County, Ariz.
A missing Wells Fargo armored van was found in the parking lot of a Sun City church.
The van's doors were locked, its engine running. The driver, John Magoch, was slumped over the wheel, shot through the head.
Wells Fargo later determined that more than half a million dollars in cash, along with checks and other valuables, was missing from the van.
Suspect Timothy Stuart Ring was eventually captured and a jury convicted him of first-degree murder, conspiracy to commit armed robbery, armed robbery, burglary and theft.
The trial judge, acting under the state plan without jury involvement, sentenced him to death.
When the Arizona Supreme Court agreed, Ring's attorneys asked the Supreme Court of the United States to intervene.
The justices heard argument in April and reversed the lower court on Monday.
"The right to trial by jury guaranteed by the Sixth Amendment would be senselessly diminished if it encompassed the fact-finding necessary to increase a defendant's maximum prison term by two years," Ginsburg said for the majority, "but not the fact-finding necessary to put him to death."
Justice Sandra Day O'Connor, joined by Chief Justice William Rehnquist, dissented.
"I continue to believe ... that the decision in Apprendi was a serious mistake," O'Connor said. " ... (the ruling) is not required by the Constitution, by history or by our prior cases."
The Supreme Court also handed down a second "son of Apprendi" case decision Monday that went the other way.
Writing for a court majority, Justice Anthony Kennedy said the federal law that makes it a felony to use a firearm during any crime of violence or drug trafficking defines a single crime.
William Joseph Harris sold marijuana out of his North Carolina pawnshop with an unconcealed semiautomatic pistol at his side.
Once convicted of the federal firearms law, it was up to a judge to decide whether Harris had brandished the weapon and would therefore have his sentence lengthened.
Kennedy and a narrow court majority that agreed to bits and pieces of his opinion upheld an appeals court ruling out of Richmond, Va., that said Harris was treated constitutionally.
Harris was sentenced to seven years in prison based on a judge's finding that he in fact brandished a weapon, Thomas said, "a fact that was neither charged in his indictment nor proved at trial beyond a reasonable doubt."
(No. 01-488, Ring vs. Arizona; and No. 10666, Harris vs. United States)