Abstract: A sliding door locking device is used to lock a sliding door, such as an overhead sliding door used in garages, storage trailers and the like. The sliding door locking device includes a door mounting member that is positioned over an edge or top portion of the sliding door and a movement restricting member that locks to the door mounting member and abuts a structural support to restrict movement of the sliding door. One example of the door mounting member includes a locking pin and a locking ring. The movement restricting member includes a locking slot for receiving and locking with the locking pin and a hasp having an aperture for receiving the locking ring. A padlock or other locking mechanism is inserted through the locking ring to lock the movement restricting member to the door mounting member. The sliding door locking device thus provides a secondary lock that locks the sliding door from the outside without having to be permanently mounted to the door or other structure.