Ahmad Rida al-Hasani – who is also an official in the city's municipal council - was taken into custody following an overnight raid, officials in al-Sadr's office said on Tuesday.
Occupation forces have refused to say why al-Hasani was seized from his home, but al-Sadr's office said it is all part of US attempts to put pressure on the leader of a minority Shia uprising.
Al-Sadr ordered his Mahdi Army militia to resist occupying troops two months ago.
But this month he agreed a ceasefire in Najaf and Karbala under pressure from other Shia religious leaders displeased with fighting in and around religious shrines.
Arrest or talks?
The US-led administration in Iraq says al-Sadr must surrender to an Iraqi arrest warrant in connection with the murder of a rival cleric in Najaf last year.
Iraqi Shia leaders say they will hold talks to find a solution to the issue.
Meanwhile, a bomb exploded early on Tuesday at the same Baghdad airport intersection at which four occupation contractors died last week.
However, Iraqi policeman Wadi al-Mini said he knew of no casualties and that a second bomb found near the road had been defused.