A magnetic valve security device prevents unauthorized opening of fire hydrant valves. The magnetic valve security device includes an inner cylindrical member having a lower end with a recess to receive the valve nut in a non-rotational relationship and an upper end with a first coupling element. The magnetic valve security device also includes an outer rotational member mounted over the inner cylindrical member for rotation with respect to the inner cylindrical member. The outer rotational member includes a second coupling element. The magnetic valve security device also includes a movable element having a non-magnetic body configured to receive a fitted magnetizable piece. The movable element is urged to a first position to decouple the first and second coupling elements. The movable element is movable to a second position to couple the first and second elements in a non-rotational relationship. The rotation of the outer rotational member rotates the inner cylindrical member to rotate the valve stem and operate the valve with the movable element in the second position.