Abstract: A locking license plate holder for mounting license plates on automobiles, trucks and trailers includes a frame having a channel formed therein dimensioned for insertion of a license plate. The frame includes a transparent cover plate provided with an elongated lock housing. Spaced pairs of aligned apertures are formed through the frame, the license plate, the cover plate and a side wall of the lock housing. A pair of retaining pins are dimensioned for insertion through a vehicle mounted license plate attaching bracket and through the pairs of aligned apertures in the license plate holder. A key actuated rotary locking cylinder is centrally mounted in the lock housing and includes a pair of transverse locking rods connected to the rotary cylinder for simultaneous extension and retraction. A pair of guide members in the housing constrains the locking rods to reciprocal linear movement. A transverse aperture is formed through each of the retaining pins for receiving the locking rods in a locked condition.