Iranian judicial spokesman Gholamhossein Mohseni-Ejheie spoke in a video statement carried by the semi-official news agency Fars News.
The identity and gender of the Iranian-American and the dates of the trial and arrest were not disclosed. The person was arrested and put on trial for "gathering information," and the sentence can be appealed, Mohseni-Ejheie said.
The US State Department called for the immediate release of all citizens held unjustly in Iran, a State Department official told CNN.
"The Iranian regime continues to detain US citizens and other foreigners on fabricated national-security related changes," the official said. "The safety and security of US citizens remains a top priority. All US citizens, especially dual nationals considering travel to Iran, should carefully read our latest travel warning."
Other Americans held in Iran
In October 2016, San Diego resident Reza "Robin" Shahini was sentenced to 18 years in prison for spying. The US-Iranian citizen was visiting family in Gorgan, Iran. He was released on bail in April to await his appeal, The Center For Human Rights in Iran said.
Siamak Namazi had been arrested almost a year before. He was the first US citizen held since the Iranian nuclear program agreement.
Four US prisoners were released last year by Iran as part of a prisoner swap granting clemency to seven Iranians indicted or imprisoned in the United States.
President's brother arrested
In the same announcement Sunday, the Iranian judicial spokesman said the brother of President Hassan Rouhani was arrested on charges of "financial irregularities."
Hossein Ferydoon has been under investigation along with others, Mohseni- Ejheie said. It is unclear how long the investigation has been ongoing.
Ferydoon can be released on bail as the investigation continues, Mohseni-Ejheie said. The amount of the bail wasn't disclosed.
CNN's Sarah Sirgany and Lauren Meier contributed to