Abstract: An anti-theft device for semi-truck trailer landing gear of the type of telescoping concentric inner and outer cylinders which inhibits the landing gear from being raised by preventing the retraction of the inner cylinder into the outer cylinder, the anti-theft device defining an elongated pin adapted to reside in an opening situated transversely through the inner cylinder and located proximate the outer cylinder, the elongated pin having a head at one end, an extending and retractable cog emerging from the shank of the elongated pin proximate the head, and a locking means interiorly to the hollow bore of the elongated pin with the lock located at the end of the elongated pin opposite the head and connected with the cog in order to extend and retract it.