The American Center for Law and Justice said Saeed Abedini was sentenced by a judge after the pastor was tried on charges of attempting to undermine the Iranian government, CNN reported Sunday.
His attorney apparently wasn't permitted to attend some of the hearings, the center said.
CNN said it couldn't independently verify the information released by the center, founded by televangelist Pat Robertson.
Abedini, who was born in Iran and lived in Idaho with his family, has been in jail in Iran since September, the group said.
"This is a real travesty -- a mockery of justice," Jordan Sekulow, the center's executive director, said in a statement. "From the very beginning, Iranian authorities have lied about all aspects of this case, even releasing rumors of his expected release."
The U.S. State Department issued a statement condemning "Iran's continued violation of the universal right of freedom of religion." It also called on "Iranian authorities to respect Mr. Abedini's human rights and release him."
Last week, Abedini provided a written statement to the judge and was questioned by prosecutors and his defense attorney, CNN said.
The judge has faced sanctions by the European Union "because of his harsh sentences for those on trial for exercising a fundamental human right," the American Center for Law and Justice said.