An anti-theft device for a towed trailer vehicle wherein a pivotable cover housing is mounted to the tongue of the trailer in spaced relation to the socket lock mechanism of a conventional ball socket attachment of a trailer hitch bar. The protective housing adapts to all standard ball and socket arrangements, does not require extensive rework of the tongue of the trailer, has adequate securing means, is low cost, and provides anti-theft protection while the trailer is in tow, as well as when the trailer is parked and/or unhitched from the towing vehicle.