Abstract: An adjustable valve locking system for valves in which the axial position of a spindle which carries a valve element is dependent upon rotation of an actuating mechanism. Such a locking system includes an annular member surrounding the valve spindle and rotatable with the actuating mechanism, a connection between the annular member and the valve spindle whereby rotation of the annular member will cause axial movement of the valve spindle, a ring threadedly engaging the outer periphery of the annular member, and a radially-movable locking bolt movable into engagement with the ring to lock the same against axial movement while preventing rotation of the annular member and any linear movement of the valve spindle to lock the same.