Abstract: Anti-theft security device for trailers of trucks is provided with a security lock integrated to a mechanism actuating the props of trailers. The anti-security device acts either mechanically on the lifting-lowering motion transmission mechanism while preventing the gears from meshing or by clocking the bevel gear by means of a metal part fitting between two teeth, or by impeding the rotation of the rotary drive bar connecting the two props. The device can also act electrically by cutting the supply of an electric motor supplied by the tractor battery and actuating the props. The mechanical and electrical locks can be combined. The device is particularly applicable to the field of road trucks of large loading capacity which are comprised of a tractor part and a trailer part articulated to each other and which may be detached, and is intended to prevent the theft of the trailer when it is not running and uncoupled.