Abstract: A gun trigger locking device comprised of two members adapted to fit around the trigger and trigger guard on a gun to prevent firing of the gun. The two members are adapted to mate and surround the trigger, preventing movement in any direction. A shaft and socket are arranged to secure the two members together and prevent the trigger locking device from being easily removed. This is accomplished by a biasing means between the shaft and the socket which requires opposite axial forces to be simultaneously applied to separate the shaft from the socket. The socket has a knob containing a threaded insert and rod extending into the socket which is externally adjustable to vary the depth of the socket. When the insert is adjusted it prevents any separation of the shaft and socket. The insert is keyed by means of an unusually shaped socket in which a separate tool or key must be inserted to adjust the insert.