The country's defence ministry said Izzat Ibrahim al-Douri was arrested in the town of Tikrit on Saturday in a joint US-Iraqi raid.
The US has not yet confirmed the arrest. Mr Douri had been the most senior figure in the former Iraqi government who was still at large.
He was sixth on the US's most wanted list. The top five have all been found.
An Iraqi government spokesman said Mr Douri was being held by the country's national guard at a secret location,
Mr Douri was reported to be very ill when he was captured, apparently at a clinic.
He was diagnosed with leukaemia several years ago and was thought to be having a blood transfusion when he was arrested.