The ruling came in an "extraordinarily brutal murder" case out of South Carolina.
It marked the fourth time that the Supreme Court has told the sometimes reluctant state courts of the requirement.
In 1994's Simmons vs. South Carolina, a Supreme Court majority said that when "a capital defendant's future dangerousness is at issue and the only sentencing alternative to death available to the jury is life imprisonment without possibility of parole, due process entitles" the defendant's lawyer to tell the jury of that fact.
The principle was rammed home in 2000's Ramdass vs. Angelone in 2000, in 2001's Shafer vs. South Carolina and again in Wednesday's ruling.
In the case decided Wednesday, William Kelly was indicted in 1996 "for an extraordinarily brutal murder, kidnapping and armed robbery, and for possession of a knife during the commission of a crime," according to Justice David Souter's majority opinion.
A jury convicted Kelly on all charges, and the trial proceeded to a separate sentencing phase.
During the proceedings, the prosecutor's description of Kelly was quite provocative.
At one point, Kelly's lawyer asked the judge to tell the jurors that if his client received a life sentence, he would be ineligible for parole.
But the prosecutor objected, saying he was not going to argue future dangerousness. He went on to call Kelly, among other things, "the butcher of Batesburg" and "bloody Billy" and spoke about his being "a little more dangerous" because of his intelligence.
Kelly's lawyer objected at that point, but the prosecutor went on to say he was speaking about Kelly's dangerousness to his victim, Shirley Shealy.
With no instruction from the judge about the life without parole alternative, the jurors sentenced Kelly to death, and the South Carolina appeals courts affirmed.
In his majority opinion Wednesday, Souter said Kelly was entitled to a jury instruction that he would be ineligible for parole under a life sentence.
The South Carolina courts had ruled that 1994's Simmons did not apply to Kelly's case or the state's current sentencing scheme, since life with the possibility of parole was an option.
But Souter said under the South Carolina sentencing scheme, a jury only makes a sentencing recommendation when there are aggravating circumstances.
And when there are aggravating circumstances, Souther said, in South Carolina the only alternatives are the death sentence or life in prison without parole.
Chief Justice William Rehnquist dissented, joined by Justice Anthony Kennedy. Rehnquist said Wednesday's decision actually departs from 1994's Simmons, which allowed prosecutors to show that a convicted murderer would be dangerous to fellow inmates, even if sentenced to life in prison.
As for Kelley's dangerousness, Rehnquist said, "I would hazard a guess that many jurors found the sheer brutality of this crime -- (Kelly) bound the hands of the victim (who was six months pregnant) behind her back, stabbed her over 30 times, slit her throat from ear to ear and left dollar bills fastened to her bloodied body -- indicative of (Kelly's) future threat to society."
Justice Clarence Thomas, joined by Justice Antonin Scalia, also dissented. Thomas said states should be allowed to have their own sentencing schemes and the Supreme Court should not try to "micromanage" them.
Wednesday's ruling eventually will allow Kelly a new sentencing trial.
(No. 00-9280, Kelly vs. South Carolina)