A locking device for a foldable backrest of a seat, has a locking housing with a handle, the handle being pivotably between locking and unlocking positions and by which a pawl can be carried between retaining and non-retaining positions, and has a blocking element which is movable by a closable lock manually between a blocking position blocking the handle and a release position releasing the handle. The blocking element is a locking bolt mounted pivotably in the locking housing between the blocking and release positions and having release and blocking recesses. A blocking pin, in the release position of the locking bolt, projects into the release recess and is freely movable between the locking and unlocking positions. The blocking pin, in the blocking position of the locking bolt and the locking position of the handle, protrudes into the blocking recess and is blocked against pivoting into the unlocking position.