The American Center for Law and Justice says the Tehran Court of Appeals on Sunday refused to reduce Saeed Abedini's prison sentence.
Abedini, who is of Iranian origin but lived in Idaho, has been jailed in Iran since September.
He was charged with attempting to undermine state security by creating a network of Christian churches in private homes. He was tried and sentenced in January.
The Idaho Statesman reports Abedini was in Iran doing humanitarian work when he was arrested.