Relative energy deficiency in sport (RED-S) is a syndrome in which disordered eating (or low energy availability), amenorrhoea/oligomenorrhoea (in women), and decreased bone mineral density (osteoporosis and osteopenia) are present. It is caused by eating too little food to support the amount of energy being expended by an athlete, often at the urging of a coach or other authority figure who believes that athletes are more likely to win competitions when they have an extremely lean body type. RED-S is a serious illness with lifelong health consequences and can potentially be fatal.

RED-S is the broader, more comprehensive name for what was formerly known as the female athlete triad (or simply the triad), which was a condition seen in females participating in sports that emphasize leanness or low body weight. As the non-menstrual components are also seen in males, the name was changed to the comprehensive term RED-S.