Abstract: A device for stabilizing a ladder for supporting it against an upright surface has an L-shaped member having a horizontally extending plate terminating in a downwardly depending section and a means for detachably securing the downwardly depending section to the upper end of the ladder. A second plate, hingedly attached to the L-shaped member, is movable between an operative position and an inoperative position. When in the operative position, the stabilizer is adapted to rest against the upright surface thereby preventing the ladder from tipping. When in the inoperative position, the second plate is parallel to the downwardly depending section of the L-shaped member. A locking means locks the second plate in the operative position, and spring maintains the second plate in the inoperative position when the stabilizer is not in use.