Weight gain is an increase in body weight. It is generally caused by the intake of excess calories, little exercise, or may be caused by a medical condition. One exception is bodybuilding, in which muscle is gained by workout routines. With a large differential diagnosis, weight gain is a prevalent complaint among adult patients.

It may be intentional or unintentional, sometimes for muscle building, or for health reasons, i.e. increasing body fat percentage to a healthy amount. Intentional reasons may include gaining weight to get to a healthy weight, bodybuilding, and recovery for eating disorders like anorexia nervosa.

See obesity for a full discussion.