Iran has curbed public executions which were common in 2005
Reports from Iran say a boy aged 17 has been hanged, in violation of UN charters signed by Tehran and official pledges not to execute minors.
Mohammad Hassanzadeh is the second juvenile offender to be hanged in Iran this year. There were seven in 2007.
He was convicted of killing a 10-year-old boy two years ago.
Rights groups say Iran used to get round commitments not to execute minors by keeping them on death row until they turn 18, but now do not appear to wait.
Human rights activists say two other juvenile offenders due to be executed on Wednesday were given a one month reprieve.
Eight men were taken for execution at Evin prison near Tehran, however, bringing the total number of executions so far this year to 128 according to Amnesty International.
Hassanzadeh was hanged in Sanandaj prison on Tuesday, Kargozaran newspaper reported on Thursday.
The paper said Iran's judiciary chief Ayatollah Mahmoud Hashemi Shahroudi had advised the local court to "settle the issue through reconciliation".
"None of our efforts to reach an agreement with the victim's family was successful and therefore the sentence was carried out," an unnamed judicial official was quoted as saying.
Under Islamic law, a victim's relatives can spare a murderer from execution by accepting blood money.
Convicted juveniles Mohammad Fadaie and Behnoud Shojaie were given another month to find an agreement with their victim's family.
A statement from the Campaign for Human Rights in Iran called executions of juveniles barbaric and tragic - saying Iran now accounts for two thirds of the under age executions in the world.
Iran is a signatory to international agreements such as the UN Convention on the Rights of the Child which outlaw juvenile executions.
One human rights group says it knows of at least 106 offenders on death row in Iran who were juveniles when they committed their crimes.
One of those, Saeed Jazee, is due to be hanged later this month, reports say.
Murder, rape, armed robbery, kidnapping and drug trafficking are all punishable by death in Iran.