Iranian lawyer Mohammad Mostafaei on Sunday said his 21-year-old client was hanged after four years in prison following his conviction for murder while still a juvenile.
Behnoud Shojai was executed in Teheran's Evin prison on early Sunday, Mostafaei said.
Shojai was only 17 when he stabbed to death another teenager in a brawl at a Teheran park.
Mostafaei said the execution of juvenile offenders contradicts Iran's international obligations and commitments.
Iran executes more juvenile offenders than any other nation - eight last year and 42 since 1990, according to Amnesty International.