Shaikh Mithal al-Hafnawy, al-Sadr's representative in the Shia holy city of Karbala, was detained in Karbala in a joint raid by occupation forces and the Iraqi National Guard on Saturday. His brother was also captured.
They were taken to a US base in the area.
"They just barged into his home at around 0300am (2300 GMT) and arrested him and his brother for no reason whatsoever," said Shaikh Raid al-Kadhimi.
An al-Sadr official in Najaf condemned al-Hafnawi's arrest and demanded he and other loyalists to the Shia leader should be released.
In April fierce clashes erupted between American forces and al-Sadr supporters after occupation troops detained an aide close to the Shia leader and closed down his publication, al-Hawza newspaper.
Al-Sadr's loyalists took control of Karbala during the months of fighting, which ended only after the intervention of senior Shia clerics and political figures. Since then there has been relative calm in the area.