Hassan Lahouti was detained by security police Sunday at Tehran airport after arriving on a flight from abroad, the Fars news agency said.
His grandfather, who heads the powerful Assembly of Experts, is still a powerful player in the ruling establishment.
A source close to the family told Fars that Lahouti, believed to be in his 20s, left the country last year days after the presidential vote in which President Mahmoud Ahmadinejad was re-elected.
The election plunged the Islamic republic into its most serious internal crisis since the 1979 revolution, when opposition supporters took to the streets in protest against the result.
Lahouti's mother, Faezeh Hashemi, was among several of Rafsanjani's relatives held briefly for joining rallies in June, when the unrest first erupted.
Rafsanjani has been criticized by Ahmadinejad supporters for what they said was his failure to give full backing to Supreme Leader Ayatollah Ali Khamenei in the crackdown on the protest movement.
Rafsanjani expressed support for Khamenei, a Shi'ite Muslim jurist who holds ultimate authority in Iran, at a meeting of the Assembly of Experts in February.
(Reporting by Hashem Kalantari; writing by Andrew Hammond; editing by Andrew Dobbie)
