The court was divided on the issue, but voted 5-4 that the death penalty for criminals aged 16 and 17 should be declared unconstitutional.
The decision affects not only those convicted in future, but about 70 prisoners already on death row for offences committed before they were 18.
Anti-capital punishment campaigners claimed the decision as a victory.
“Now the US can proudly remove its name from the embarrassing list of human rights violators – that includes China, Iran, and Pakistan – that still execute juvenile offenders,” said William Schulz, executive director of Amnesty International USA.
