The State Department said it had confirmed reports of Saeed Abedini's sentencing with the family's attorney and called for his release.
"We condemn Iran's continued violation of the universal right of freedom of religion and we call on the Iranian authorities to respect Mr. Abedini's human rights and release him," State Department spokesperson Darby Holladay said.
Hollaway said Abedini's attorney had only one day to present his defense and the department was deeply concerned about the fairness and transparency of his trial.
The American Center for Law and Justice, a conservative lobbying group, said Abedini was convicted by Judge Pir-Abassi of Branch 26 of the Iranian Revolutionary Court for threatening Iran's national security through his leadership in Christian house churches.
It said Abedini, a 32-year-old father of two, became a U.S. citizen in 2010 through marriage to his American wife Naghmeh.
He was ordained a minister in 2008 through the American Evangelistic Association and had been traveling back and forth freely between the U.S. and Iran for three years until this past summer, when he was placed on house arrest. He was imprisoned in September.
The ACLJ, which is representing Abedini's U.S.-based wife and children, said the pastor had been beaten and tortured during his time in prison, raising questions about his medical condition.
It said Abedini and his attorney were only allowed to attend one day of his trial, which started on January 21.
"This is a real travesty - a mockery of justice," said ACLJ's Executive Director Jordan Sekulow. "From the very beginning, Iranian authorities have lied about all aspects of this case, even releasing rumors of his expected release. Iran has not only abused its own laws, it has trampled on the fundamentals of human rights."
In a statement, Naghmeh Abedini said she is "devastated" for her husband and her family - a six-year-old daughter and a four-year-old son.
"We must now pursue every effort, turn every rock, and not stop until Saeed is safely on American soil," she added.