A door latch bolt locking device for use with a latch bolt having a transverse groove or kerf in its top surface and which will positively prevent the withdrawal of the latch bolt. The device comprises a slideable member adapted to be substantially flush with the intersecting edge surface of the door and the inner surface of the door. The slideable member will have a first portion which will be substantially flush with the inner surface of the door and an integral tongue portion at a right angle to the flat portion and adapted to be substantially flush with the edge surface of the door. The locking device is also provided with means for allowing the slideable member to be moved through a limited vertical distance in a substantially flush relationship with the door surfaces so that the tongue member can move into and out of the kerf provided in the latch bolt to thereby lock the door.In a preferred embodiment, the locking device is provided with means for positively maintaining the slideable member in a bolt-locked position when subjected to vertical forces.