An improved modular trigger assembly for an arc welding gun includes a trigger housing pivotably mounted to a welding gun. A switch is mounted inside the trigger housing and is actuated by movement of the trigger housing. The trigger housing also includes a slideable locking crescent having a first end near a finger surface of the trigger housing. This locking crescent is so arranged that pressure on the first end surface serves both to depress the trigger and to lock the trigger in the depressed position.