Abstract: A magnetic security device for preventing unauthorized opening of fire hydrant valves includes an inner cylindrical member which is fixed to a nut on the valve stem and an outer rotatable member which is coupled to the inner cylindrical member by a magnetizable element. The magnetizable element is attracted to move from an uncoupled to a coupling mode by a reaching permanent magnet integral with a special operating wrench. In accordance with one embodiment of the invention, an intermediate sleeve is provided which is coaxial with a nylon sleeve disposed around the outer surface of the inner cylindrical member. In addition, the inner cylindrical member has a knurled portion which engages the surface of a recess surrounding the valve operating nut so that if the outer operating member is tilted with respect to the inner operating member, the inner operating member binds with the wall of the recess, preventing rotation of the inner operating member and of the valve operating nut attached thereto.