Abstract: This invention is designed to lock a trailer coupling, which includes a trailer hitch of the lever type and a ball-type connector. The coupling is locked by a locking-ball which is constructed by modifying a conventional ball to conceal the locking element, and how it provides the locking function. The locking-ball includes a substantially spherical ball in which a hardened steel bolt is threadedly disposed axially therewithin for vertical axial movement of the bolt through the upper surface of the locking-ball. The locking-ball is inserted in the socket of the trailer hitch, and the lever is closed in the "down" position so as to actuate a cam against a jaw, which jaw in cooperation with the socket, snugly holds the ball in position. The bolt is then advanced vertically by a key in mating relation with the lower end or head of the bolt, until the upper end of the bolt is forced against the inner surface of the socket, and the ball is forcibly biased within the socket against the jaw.