A golf swing training device that fits on the inner down swing arm and optionally it has a thumb restraint extension. The device comprises a shell contoured to fit the inner arm from the wrist to the lower portion of the upper arm which is strapped to the arm. The device cocks the arm slightly but fully extends the arm for the back swing and down swing and aids the golfer in forming the right habit by muscle memory to extend the arm during the golf swing. The device is flexible laterally so that a snug fit between the device and the arm can be obtained. The device can twist to accommodate the twisting of the arm during the golf swing. The device resists bending of the forearm about the elbow to keep the forearm extended. The thumb extension inhibits flexure and side to side movement of the thumb during the golf swing. The thumb extension can be detachably attached to the shell.