A rotatable armrest contains a body, a connecting mount, a joining member, and a cover. The body includes an inner rod and an outer rod. The inner rod has a rotating seat, a through hole, a first groove, a second groove, and two retaining shoulders. The connecting mount includes a locking rib and a column, the column has a slot and two notches. The joining member includes a first segment mounted in the through hole of the inner rod and a second segment fixed in the first groove of the inner rod, the first segment has two engaging blocks formed on an outer peripheral side thereof and a lock protrusion formed therein and accommodated in the slot, the second segment has a trench for receiving the cover, and the trench has a cutout which has two threaded orifices passing through the lock protrusion of the joining member.