"The Court makes clear that Louisiana's experiment with the death penalty for rape ran afoul of the United States Constitution," Ben Cohen, of the New Orleans-based Capital Appeals Project, said.
The organization has represented Kennedy for the past four years.
"Given the Court's decision, we can only hope that the money that Louisiana has been spending drafting and defending this anomalous and unconstitutional statute will be reallocated to efforts at treatment for victims of sexual abuse and for measures that actually reduce the risk of such abuse in our communities," Cohen said.
Representatives of Louisiana had argued strenuously for the law.
"In this instance, the victim was an 8-year-old girl who was asleep in her own bed in the morning and found her 300-pound stepfather violently physically raping her," Texas Solicitor General Ted Cruz said during oral arguments.
But some victims of child rape also supported Kennedy. Jody Plauche, now 36, who was raped and kidnapped as a child, says that the possibility of the death penalty adds too much burden to the child.
"A child who's been raped has been through enough," he said.
He points out that the offenders are usually a trusted adult and he worries the children will feel "extra trauma" if they know that the offender might die if the child reports the crime.