Abstract: A locking device to inhibit unauthorized operation of a fire hydrant includes a locking plug having a bottom opening that fits over an actuating nut of the hydrant. The locking plug has a cylindrically-shaped body formed with an upper actuator nut. A peripheral groove in a sidewall of the plug body is located above a set of inward extending, threaded bores for set screws to affix the locking plug to the hydrant actuating nut. Fitting over the plug is a cap having a passageway that intersects tangentially with the plug groove. A locking pin on a hasp bracket then is inserted into the cap passageway to form an interference fit with the plug groove. Rotating the bracket upward positions a staple post on the cap in a slot in the hasp bracket. A padlock then secures the position of the hasp bracket to inhibit access to the plug actuator nut and operatively connected hydrant actuating nut.