The ruling also invalidates laws on the books in five other states that allowed executions for child rape that does not result in the death of the victim.
However devastating the crime to children, Justice Anthony Kennedy wrote in his majority opinion, "the death penalty is not a proportional punishment for the rape of a child." His four liberal colleagues joined him, while the four more conservative justices dissented.
There has not been an execution in the United States for a crime that did not also involve the death of the victim in 44 years, a factor that weighed in Kennedy's decision.
Rape and other crimes "may be as devastating in their harm, as here, but 'in terms of moral depravity and of the injury to the person and to the public,' they cannot be compared to murder in their 'severity and irrevocability,"' Kennedy said, quoting from earlier decisions.
The victim in the case decided Wednesday was an 8-year-old girl raped by her stepfather at their home in Harvey, La., outside New Orleans.
Angry Louisianans who backed the law said the court was out of touch.
"The opinion reads more like an out-of-control legislative debate than a constitutional analysis," said Louisiana Gov. Bobby Jindal, a Republican. "One thing is clear: The five members of the court who issued the opinion do not share the same 'standards of decency' as the people of Louisiana."
With the court already on record this term reaffirming the constitutionality of capital punishment in a case dealing with lethal injection, Kennedy dwelt at length on the need to limit the death penalty to the most heinous killings.
The decision allows death sentences to continue to be imposed for crimes such as treason, espionage and terrorism, which Kennedy labeled as crimes against the state.
The Supreme Court banned executions for rape in 1977 in a case in which the victim was an adult woman.
Forty-four states prohibit the death penalty for any kind of rape, and five states besides Louisiana have allowed it for child rapists - Georgia, Montana, Oklahoma, South Carolina and Texas.
The court struggled over how to apply standards laid out in decisions barring executions for the mentally retarded and people younger than 18 when they committed murder. In those cases, the court cited trends in the states away from capital punishment.
In this case, proponents of the Louisiana law said the trend was toward the death penalty, a point mentioned by Justice Samuel Alito in his dissent.
"The harm that is caused to the victims and to society at large by the worst child rapists is grave," Alito wrote. "It is the judgment of the Louisiana lawmakers and those in an increasing number of other states that these harms justify the death penalty."
But Kennedy said the absence of any recent executions for rape and the small number of states that allow it demonstrate "there is a national consensus against capital punishment for the crime of child rape."
Kennedy acknowledged that the decision had to come to terms with "the years of long anguish that must be endured by the victim of child rape."
Still, he concluded that in cases of crimes against individuals, "the death penalty should not be expanded to instances where the victim's life was not taken."
He also cited arguments made by social workers and others that children and their families might not cooperate with authorities if a death sentence could result against the rapist. In many cases, including the one before the court, the victim and rapist are related.
The author of the Louisiana law, former Republican state Rep. Pete Schneider, said even opponents of the death penalty told him they would kill anyone who raped their children. "When are you going to have the courage to stand up for what's right for all of the people - but especially the children under 12 that have been brutally raped by monsters?" Schneider demanded, directing his comments to the justices in Wednesday's majority.
The last executions for crimes other than murder took place in 1964, according to a database maintained by the Death Penalty Information Center.
Ronald Wolfe, 34, died in Missouri's gas chamber on May 8, 1964, for rape. James Coburn was electrocuted in Alabama on Sept. 4 of that year for robbery.
The case before the court involved Patrick Kennedy, 43, who was sentenced to death for the rape of his 8-year-old stepdaughter in Louisiana.
Kennedy was convicted in 2003. The girl initially told police she was sorting Girl Scout cookies in the garage when two boys assaulted her.
Police arrested Kennedy a couple of weeks after the March 1998 rape, but more than 20 months passed before the girl identified him as her attacker.
His defense attorney at the time argued that blood testing was inconclusive and that the victim was pressed to change her story.
The Louisiana Supreme Court upheld the sentence, saying that "short of first-degree murder, we can think of no other non-homicide crime more deserving" of the death penalty. State Chief Justice Pascal Calogero noted in dissent that the U.S. high court already had made clear that capital punishment could not be imposed without the death of the victim, except possibly for espionage or treason.
The girl's mother was reached by The Associated Press following the court's decision Wednesday. "We don't talk about that," she said and hung up.
A second Louisiana defendant, Richard Davis, was given the death penalty in December for repeatedly raping a 5-year-old girl in Caddo Parish.
Local prosecutor Lea Hall told jurors: "Execute this man. Justice has a sword and this sword needs to swing today." Both men will get new sentences.
The case is Kennedy v. Louisiana, 07-343.