State television quoted spokesman Gholamhossein Mohseni Ejei as saying Hossein Fereydoun, who is also a close adviser to Rouhani, was summoned for questioning, on Saturday, in a case in which he faces unspecified financial charges. He was later detained.
"If Fereydoun arranges bail, he will be released," Mohseni Ejei was quoted as saying. "He has been heard several times, and others have also been questioned, and some of them are being held ... and the case continues."
No details were available on the amount of bail he needs to pay.
Some Rouhani supporters have interpreted the charges against Fereydoun, a senior diplomat who took part in the talks that led to a 2015 nuclear deal between Iran and world powers, as a move by the hardline judiciary to discredit the president.