A BOSU Balance Trainer, or BOSU ball as it is often called, is a fitness training device, invented in 1999 by David Weck,^{[citation needed]} consisting of an inflated rubber hemisphere attached to a rigid platform. It is also referred to as the "blue half-ball", because it looks like a stability ball cut in half. The name is an acronym which stands for "BOth Sides Up" a reference to the two ways a BOSU ball can be positioned. The device is often used for balance training. When the dome side faces up, the BOSU ball provides an unstable surface while the device remains stable. This combination of stable/unstable allows a wide range of users, from the young, elderly, or injured to the elite level athlete. With the dome side up, the device can be used for athletic drills and aerobic activities. As the name suggests, the device can also be flipped over so that the platform faces up. In this position, the device is highly unstable and can be used for a wide array of exercises as well.

In a scientific experiment conducted at Eastern Illinois University in 2009, 12 men performed various physical exercises (back squat, deadlift, overhead press, and curl lifts) with and without the BOSU ball. Using the BOSU ball did not create a real difference in the activity of their muscles. Therefore, it was concluded that the BOSU ball did not bring a significant improvement for these physical exercises and that these exercises performed on stable grounds were as efficient as the ones performed on the BOSU ball. A second study which concentrated on single-leg stance shows very similar results (no difference in muscle activity with and without the BOSU ball).