A tool for setting an instrument sound post preferably includes a bent tube, one or more string or polymer lines, a cradle end that grasps the sound post, and an internal locking mechanism to hold the sound post against the tool. In one embodiment, a single line is used and a closed loop at the cradle end of the sound post setting tool holds the sound post in place. In this embodiment, a double-chamfer threaded tube construction is preferably used to facilitate line threading. In another embodiment, the cradle end includes a loop and a ball to grasp the sound post. In yet another embodiment, the cradle end includes two balls to grasp the sound post. The tool provides an improved way to install a sound post inside a stringed instrument.