Abstract: A trailer tongue locking device includes a hitch bar coupled to the towing vehicle which supports an upwardly extending ball member and a downwardly extending lock nut terminating in a downwardly tapered chamfered surface. An upper and lower plate are securable to the tongue portion of the trailer locking device in an anti-theft locking attachment. The lower plate defines an elongated slot which is aligned beneath the socket portion of the trailer tongue and which receives the chamfered surface of the lock nut. The cooperation of the lock nut, the chamfered surface and the socket together with the ball member provides a secure safe attachment between the towing vehicle and trailer while the degree of movement of the chamfered surface of the lock nut within the slot permits the necessary relative motion within the trailer tongue locking device to accommodate normal towing circumstances.