Provided is a valve timing adjustment system having improved control for achieving a target rotational phase. The valve timing adjustment system includes: a displacement mechanism unit that displaces a rotational phase of a camshaft relative to a crankshaft of an internal combustion engine; a locking mechanism unit that locks the rotational phase at an intermediate locked phase positioned within a displacement range of the rotational phase; a hydraulic pathway that hydraulically drives the displacement mechanism unit and the locking mechanism unit; and a control unit including a control system that controls operations of the hydraulic control valve. The control unit changes a temporal responsiveness of the control system based on a displacement force that displaces the rotational phase.