Ahmed Arif, the spokesman for the Muslim Brotherhood was arrested Thursday, state-run television al-Masriya reported.
He's the latest key figure in the group to be taken into custody by Egypt's interim military government.
On Tuesday, the military arrested Mohamed Badie, the group's spiritual leader, and Safwat Hegazy, a preacher and hardline supporter of the movement. They were among about 100 members of the Muslim Brotherhood taken into custody Tuesday.
Ousted President Mohamed Morsy has been under detention since early July.