A retrofittable cap locking device for removably fitting over the stem of a fuel tank such that the cap of the stem cannot be removed. The retrofittable cap locking device includes a base portion. The base portion has an opening therein for receiving the stem of a tank designed for holding fuel and oil. The base portion has a peripheral edge. A cover portion comprises a plate having a top side, a bottom side and a peripheral wall extending therebetween. The bottom side has a flange thereon. The flange has a size adapted to fit around the cap of the stem. A first rod has a first end fixedly coupled to the peripheral wall and a second end hingedly coupled to the peripheral edge. The first rod is orientated generally perpendicular to a plane of the plate such that the rod extends away from the bottom side of the plate. A second rod is fixedly coupled to the peripheral wall. The second rod extends in the same direction as, and is orientated generally perpendicular to, the first rod. The first rod has a free end adapted to be releasably lockable to the base portion.