A sealing device for a camshaft adjuster which is driveable using a belt and has a distributing valve with a piston, wherein the sealing device has a first securing portion rigidly arrangeable on the camshaft adjuster, a second securing portion rigidly arrangeable on the piston side, and a flexible membrane portion arranged between the first securing portion and the second securing portion, and a camshaft adjuster having a having a stator which, based on a combustion engine, is driveable using a belt, and a rotor with limited twistability relative to the stator, wherein a twisting can be effected hydraulically, and a distributing valve with a piston for the purpose of freeing up or blocking the pathway for a hydraulic medium or altering a through-flow direction, wherein the camshaft adjuster has a sealing device; of the type whose first securing portion is rigidly arranged on the camshaft adjuster and whose second securing portion is rigidly arranged on the piston side.