Luay Khayrallaha was taken into custody Friday, said the U.S. Central Command.
He is the brother of Saddam's wife, a companion of Saddam's son, Odai, and a representative of the former regime's intelligence and security apparatus, Central Command said in a statement.
Defense Secretary Donald H. Rumsfeld has said there is a list of some 200 former regime officials who coalition troops are looking for.
According to a revised list of Iraq's 55 most wanted former leaders, posted on the command's Web site Monday, U.S.-led forces have seized or won the surrenders of 22. One member of the list, Ali Hasan "Chemical Ali" Al-Majid is believed to have been killed.
CentCom has said they hope the Iraqis in custody — which include high-ranking figures like Deputy Prime Minister Tariq Aziz and science adviser Amir al-Saeed — would help them accomplish two avowed goals of the Iraq mission: bringing Saddam Hussein to justice and locating weapons of mass destruction.
Saddam, whom U.S. warplanes twice tried to kill, has not been found and it is not clear if he is alive or dead, although reports suggest Aziz said he survived both the attacks.
The search for weapons continues, with administration officials now saying Iraq may have destroyed its alleged stocks of illegal arms in the months leading up to the war.
Two suspected mobile biological weapons labs have been located and are undergoing tests, but no other finds have been made of the large stockpiles Iraq was supposedly hoarding.
So far, Iraqi leaders in custody have maintained that Iraq had no weapons of mass destruction, according to published reports.