A coupler lock for securing a device within a sleeve of a vehicle hitch to prevent unauthorized removal of the device which can include a hitch ball insert, a cargo platform, bike carrier or similar accessory designed to be attached to the vehicle. The coupler lock includes a lock housing removably attached to one end of a lock bar configured to pass through apertures within the hitch sleeve and insert. The lock housing has a plurality of ball bearings radially movable in and out of an annular groove to lock and release the bar. A locking cup longitudinally movable within the lock housing in response to movement of the lock cylinder retains the bearings within the groove or releases the bearings from removal of the lock bar from the lock housing.