Abstract: A garage door-opener security system which automatically positively blocks any unauthorized opening of a sectional garage door that travels, guided by rollers, in a pair of flanking metal guide tracks. The security system is interconnected with a standard, automatic, garage door-opening mechanism that includes an electric drive motor. When the door reaches its fully closed position, a locking device, mounted adjacent one of the flanking tracks, physically inserts a locking pin into the trackway in which the rollers move, which pin is located a few inches above the roller next below. The locking pin is closely confined by a surrounding aperture which prevents its upward deflection or twisting, thus making it impossible for the roller to move therepast in the trackway and positively preventing the raising of the door more than a few inches.