In a 5-4 vote, the court said the Louisiana law allowing the death penalty to be imposed in such cases violates the Constitution's ban on cruel and unusual punishment.
"The death penalty is not a proportional punishment for the rape of a child," Justice Anthony Kennedy wrote in his majority opinion. 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.
Patrick Kennedy, 43, was sentenced to death for the rape of his 8-year-old stepdaughter in Louisiana. He is one of two people in the United States, both in Louisiana, who have been condemned to death for a rape that was not also accompanied by a killing.
The Supreme Court banned executions for rape in 1977 in a case in which the victim was an adult woman.
Forty-five states ban the death penalty for any kind of rape, and the other five states allow it for child rapists. Montana, Oklahoma, South Carolina and Texas allow executions in such cases if the defendant had previously been convicted of raping a child.
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 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 also 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, Kennedy concluded that in cases of crimes against individuals - as opposed to treason, for example - "the death penalty should not be expanded to instances where the victim's life was not taken."
The decision does not affect the imposition of the death penalty for other crimes that do not involve murder, including treason and espionage, he said.
