Abstract: An anti-theft device is provided which, when used on a trailer coupler, locks on to the trailer coupler and covers the receptacle of the coupler, thereby preventing the insertion of a hitch ball or other towing means into the receptacle. Locking engagement is achieved by a retaining member being moved and held in a position inside the receptacle of the coupler by the operation of a lock housed in the body of the anti-theft device. In another embodiment of the invention locking engagement is achieved by the retaining member being moved to a position inside the coupler receptacle through operation of a lever independent of the retaining member and being held in such position through operation of a lock housed in the body of the anti-theft device. No tools or auxiliary parts are required to attach or remove the device.