"This is another defeat for the pro-death penalty forces on the court and in the country," said CBSNews.com Legal Analyst Andrew Cohen.
The high court, by a 7-2 vote, overturned its 1990 decision that upheld the Arizona capital sentencing law that gives sole responsibility to the judge to make factual findings necessary to sentence a convicted murderer to death.
The court held that a sentence imposed by a judge violates a defendant's constitutional right to a trial by jury. Juries, not judges, must consider all factors that lead to a death sentence, the court found.
"This has enormous impact on death rows in America. Hundreds and hundreds of convicted murderers now may have their sentences reduced from death to life without parole and states, which had gone to judge-sentencing in death penalty cases, now have to give those decisions back to jurors," said Cohen.
In some states juries determine guilt or innocence, but a judge then bases the sentence on aggravating factors, such as the heinous nature of a murder or whether it was committed for monetary gain.
Monday's ruling turned on the Constitution's guarantee of a jury of one's peers and a Supreme Court ruling two years ago that struck down another kind of sentence determined by a judge instead of a jury.
Justice Ruth Bader Ginsburg, writing for a majority that included an unusual alliance of conservative and liberal-leaning justices, said the court's 2000 ruling in a case called Apprendi v. New Jersey cannot be reconciled with the death penalty sentencing laws in Arizona and four other states in which one or more judges impose the sentence.
The Apprendi case concerned a judge's ability to lengthen a sentence by two years if a crime was determined to be a hate crime. The high court struck down that sentencing law.
"The right to trial by jury guaranteed by the Sixth Amendment would be senselessly diminished if it encompassed the factfinding necessary to increase a defendant's sentence by two years, but not the factfinding necessary to put him to death," Ginsburg wrote. "We hold that the Sixth Amendment applies to both."
The ruling represented a victory for Timothy Stuart Ring, who was sentenced to death by an Arizona judge for the 1994 killing of an armored van driver in Phoenix.
After the verdict, the jury was discharged. The judge at a separate hearing said Ring deserved the death penalty after finding two aggravating circumstances - committing the murder for financial gain and carrying it out in an especially heinous way.
Chief Justice William Rehnquist and Justice Sandra Day O'Connor dissented. O'Connor said she would overrule the 2000 decision rather than overruling the 1990 ruling that upheld Arizona's death penalty law.
The justices, sharply divided, ruled that judges can lengthen the prison sentences of people who use guns while committing other crimes, even if the defendant hasn't been convicted of any charge specifically involving the weapon. With the 5-4 decision, justices avoided a ruling that could have threatened prison sentences of thousands of inmates and put in doubt sentencing laws in nearly every state.