A security lock for a door is provided having a bracket on the interior side of the door with a locking bolt mounted in the bracket. The bolt is adapted for movement between a raised, unlocked position above a cavity on the floor below the bracket and a lowered, locked position where the bolt is in the cavity to prevent the door from being opened. A bolt restraint is mounted on the bracket and cooperates with the bracket to maintain the bolt in its unlocked position. The bolt restraint, for example, a spring-loaded ball that engages an annular recess in the bolt, releases the bolt into its locked position when a downward force is applied to the bolt. The locking bolt has an enlarged flange that stops movement of the bolt into the cavity beyond a predetermined location on the bolt.