Abstract: A theft deterrent trailer hitch lock to obstruct removal of the socket portion of a trailer hitch from engagement with a trailer hitch ball mounted on a tow bar secured to a towing vehicle. The device includes a right angular-shaped base having a first flange underlying the hitch ball and bolted to the tow bar by bolts having an upstanding end portion projecting above the upper limit and on opposing sides of the trailer hitch socket portion. The upstanding flange of the angle base is pivotally connected with a top member vertically movable toward and away from the upper limit of the trailer hitch socket portion and having apertures therein receiving the upper ends of the base flange bolts. A lockable bar, slidably supported by the top member along its edge opposite the pivotal connection, is provided with an aperture mating with an aperture in the top member for receiving the bolt of a lock.