General Hossam Mohammed Amin, the former head of Baghdad's weapons monitors, is in coalition custody, US military officials say.
General Amin defended Baghdad's arms programmes
No details have been given as to whether he was captured or surrendered to US-led forces now in control of Iraq.
General Amin was a key contact for the United Nations arms inspectors sent to Iraq and was a spokesman for the regime's assertions that it had no banned weapons.
He is listed at 49 on the US' list of 55 most-wanted figures of Saddam Hussein's administration and features as the six of clubs on the playing cards issued to US troops hunting regime leaders.
US Central Command issued a statement saying only that General Amin, the head of Iraq's National Monitoring Directorate, was "under coalition control".
His is the latest detention of a regime leader who is better known in the West, following Thursday's apprehension of former Deputy Prime Minister Tariq Aziz.
Correspondents say General Amin could hold key information not just about weapons programmes, but also about Saddam Hussein's inner circle.
HOSSAM MOHAMMED AMIN
13th regime leader announced to be in US custody
One of few officials allowed to speak publicly about weapons of mass destruction
Believed to be close to Saddam Hussein's son Qusay
He was reported to have very good connections to the top leadership through close relations with the former president's son, Qusay, and his private secretary, Abed Hamoud al-Tikriti.
He was apparently targeted by US intelligence agents urging him to defect when he travelled to Vienna for talks with the UN's inspectorate last year.
In his official role, General Amin held press conferences detailing sites and projects checked by the UN weapons inspectors in the months before the war, always pointing out they had found no chemical, biological or nuclear arms programmes.
He warned that he expected the US to "fabricate" evidence to justify an invasion.
He and presidential adviser General Amir al-Saadi were also tasked with rebutting point by point the evidence presented to the UN Security Council by US Secretary of State Colin Powell in February.
General Saadi surrendered to coalition forces in Baghdad on 12 April.