In a 5-4 vote, the court said the Louisiana law allowing the death penalty to be imposed in child-rape cases violates the Constitution's ban on cruel and unusual punishment.
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 had been condemned to death for a rape that was not also accompanied by a killing.
"The death penalty is not a proportional punishment for the rape of a child," Justice Anthony Kennedy wrote in his majority opinion.
Justice Kennedy was joined by Justices Ruth Bader Ginsburg, Stephen Breyer, David Souter and John Paul Stevens in the opinion; Chief Justice John Roberts, Justices Samuel Alito, Antonin Scalia and Clarence Thomas dissented.
There has not been an execution in the United States in 44 years for a crime that did not also involve the death of the victim. The Supreme Court banned executions for rape in 1977 in a case involving an adult female victim.
Wednesday's decision does not affect the imposition of the death penalty for other crimes that do not involve murder, including treason and espionage, Kennedy said.
Kennedy concluded that in cases of crimes against individuals -- as opposed to treason -- "the death penalty should not be expanded to instances where the victim's life was not taken."
"The harm that is caused to the victims and to society at large by the worst child rapists is grave," Justice Alito wrote in dissent. "It is the judgment of the Louisiana lawmakers and those in an increasing number of other states that these harms justify the death penalty."
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."
The Supreme Court also reduced punitive damages for victims in the Exxon Valdez spill.
The court, divided 5-3, with Justice Alito taking no part in the case because he owns Exxon stock, cut the $2.5 billion punitive damages award in the 1989 Exxon Valdez disaster to $500 million.
The court ruled that victims of the worst oil spill in U.S. history may collect punitive damages from Exxon Mobil Corp., but not as much as a federal appeals court determined.
Justice Souter wrote for the court that punitive damages may not exceed what the company already paid to compensate victims for economic losses, about $500 million compensation.
Exxon asked the high court to reject the punitive damages judgment, saying it already has spent $3.4 billion in response to the accident that fouled 1,200 miles of Alaska coastline.
Nearly 33,000 Alaskans are in line to share in the award, about $15,000 a person. They would have collected $75,000 each under the $2.5 billion judgment.
In dissent, Justice Stevens supported the $2.5 billion figure for punitive damages, saying Congress has chosen not to impose restrictions in such circumstances.
In another ruling, the Supreme Court also said that a convicted killer deserves a new trial because jurors heard testimony that should have been excluded. His ex-girlfriend made the statements shortly before he killed her.
The justices, in a 6-3 vote, reaffirmed the rights of criminal defendants to confront witnesses against them, even in cases where the defendant is responsible for the witness' absence.
The issue arose in the case of Dwayne Giles, arrested in the shooting death of Brenda Avie in 2002, several weeks after she told police that Giles had assaulted her and threatened to kill her.