An anti-theft device is disclosed for use on a vehicle employing air-operated parking brakes. The device is shaped for disposal about the parking brake control knobs when engaged. The device includes a pair of mating halves formed for surrounding and enclosing the control knobs while in an engaged position. A first of the halves includes a tongue having an opening therein and a second of the halves includes a channel for receiving the tongue. A lock is provided with a locking pin being disposed in the second half wherein the locking pin engages the opening in the tongue in order to secure the first and second halves together. Also, an adjustment mechanism is provided for tailoring the device to minor differences between the brake control knobs from one vehicle to another.