Prison warden Mohammad Zeinali told the government-run news agency Fars that the Ahar Penal Complex had a capacity of 50 prisoners but was currently housing 235 inmates.
Ahar is situated in Irans Azeri-dominated East Azerbaijan Province.
Earlier this month, the chief of Irans prisons Ali Akbar Yessaghi told the state-run news agency ISNA that a prison in the south-western city of Abadan with a maximum capacity of 70 people was housing some 500 prisoners.
Yessaghi also said that some 600,000 Iranians were imprisoned every year.