A core strengthening device includes a tray and a ball. The tray has first and second opposing sides. The first side of the tray has a recess formed therein. A ball is selectively positionable at least partially within the recess in the tray. The ball has a first hemisphere and a second hemisphere. The first hemisphere has a first radius and the second hemisphere has a second radius that is different than the first radius. The core strengthening device provides instability to exercises, which requires a user's core muscles to work harder to perform the exercises and maintain balance, which strengthens the user's core muscles.