The man described as Saddam Hussein's enforcer has been captured in a joint US-Iraqi raid in the north of the country, Iraq's defence ministry says.
Izzat Ibrahim al-Douri was apparently arrested on Saturday while receiving treatment at a clinic near Tikrit.
Officials said 70 of his supporters were killed and dozens more arrested as they tried to prevent his capture.
Mr Douri had been the most senior figure in the former regime still at large, and the most wanted.
He is now said to be in American custody, though the US military has not yet confirmed his arrest.
Mr Douri was Saddam Hussein's number two in the Revolutionary Command Council, and was sixth on the list of 55 most wanted members of the regime. The top five have all been captured or killed.
He is accused of financing insurgent groups, and has a $10m price tag on his head.
The BBC's Paul Wood in Baghdad says it is not clear whether anyone has claimed the reward - in other words, whether he was betrayed by someone who was part of his inner circle, as were Saddam Hussein's two sons.
Tikrit is his home town, as well as that of Saddam Hussein, and our correspondent says he would have been hoping to rely on family or tribal connections there to stay at large.
Reuters news agency reported that Shia Muslims in Baghdad fired guns into the air in celebration at the capture. Most members of the Baathist ruling class were Sunni Muslims.
Mr Douri was one of the key plotters who brought the Baathist Party to power in 1968.
Twenty years later, he held a senior post on the committee responsible for northern Iraq when chemical weapons were used, killing thousands of Kurds.
War crimes charges have been issued against him in Austria.