Hossein Fereydoun, who is also a close confidante of the president, has vowed to appeal the sentence, local media reported on Saturday.
“This person [Hossein Fereydoun] was found not guilty on some charges, while he was sentenced to prison on other accusations,” Hamidreza Hosseini, a judiciary official, was quoted as saying by state news agency IRNA.
The trial of Fereydoun, along with six co-defendants, began in February without the judiciary giving details of the charges.
He had initially been held in 2017, on financial crime charges, before being released on bail.
Fereydoun, responding to the court’s decision on Saturday, said he rejected the ruling.
“I strongly and categorically reject allegations against me in the court and some of the media, and I’m protesting,” he was quoted as saying by the semi-official news agency ISNA.
Fereydoun was a senior diplomat who took part in talks that led to the 2015 nuclear deal between Iran and world powers.
Some supporters of Rouhani, who was born Hassan Fereydoun before adopting his new surname, view the charges against his brother as a move by the judiciary to discredit the president.
The judiciary has denied having any political motivation in cases it tries.
Al Jazeera and news agencies