A sealing device (12) for a camshaft adjuster (1) which is driveable using a belt and has a distributing valve (8) with a piston, wherein the sealing device (12) has a first securing portion (13) rigidly arrangeable on the camshaft adjuster (1), a second securing portion (14) rigidly arrangeable on the piston side, and a flexible membrane portion (15) arranged between the first securing portion (13) and the second securing portion (14), and a camshaft adjuster (1) having a having a stator (3) which, based on a combustion engine, is driveable using a belt, and a rotor (4) with limited twistability relative to the stator (3), wherein a twisting can be effected hydraulically, and a distributing valve (8) 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 (1) has a sealing device (12) of the type whose first securing portion (13) is rigidly arranged on the camshaft adjuster (1) and whose second securing portion (14) is rigidly arranged on the piston side.