Abstract: A king pin lock for preventing unauthorized movement of a trailer when detached from a tractor includes a hemispherical boby member having a flat planar face which faces the bottom surface of the trailer, when installed. A cylindrical trailer king pin has a circumferential groove for engagement into the fifth wheel of a towing tractor vehicle. The king pin lock has an aperture dimensioned to allow insertion of the trailer king pin. A pair of arcuate locking arms are pivotally mounted for radial movement within the hemispherical body member into and out of engagement with the king pin groove. A key actuated cam mechanism moves the locking arms between the lock and unlock positions. The locking mechanisms are assembled on a removable locking shell which may be replaced if necessary.