Abstract: The present invention relates to an anti-theft device for fuel tanks, wherein the anti-theft device comprises a pipe to be secured to a fuel inlet, wherein the pipe comprises attachment means at its upper portion, furthermore the pipe comprises at least one opening disposed in the side wall of the pipe, wherein the pipe is closed at a lower portion and that said pipe accommodates a substance, wherein said substance is adapted, in the event of a fuel theft attempt, to be carried with the fuel into the pump used during the theft attempt and thereby prevent continued pumping with it, and that the substance is further adapted to be incapable of leaking into the actual fuel tank.