A trigger lock is provided with a shield member having a U-shaped channel forming a first plate and a second plate spaced apart from the first flange, with a web connecting the first and second plates. The shield member has a peripheral contour that is complementarily shaped to closely match the transition contour of a firearm, in the region of the firearm frame transitioning between the thick frame and the thin trigger guard. The shield member is fitted over the trigger guard, with the trigger guard situated within the U-shaped channel. A plug is inserted through an aperture in the first plate, through the trigger guard, and through a second aperture in the second plate. The plug is secured within the shield member by a lock attached to the end of the plug protruding from the second aperture, thus preventing access to the trigger and preventing removal or substantial movement.