This article focuses on two popular sports that can put male athletes at risk of developing an eating disorder: bodybuilding and running. Bodybuilders concentrate on gaining muscle mass and runners on leaning body mass. Based on the trans-contextual model of motivation, this study aimed to better understand the psychological mechanisms underlying eating disorders in these athletes. In all, 272 male bodybuilders and 217 male runners completed measures of sport motivation, theory of planned behavior variables (i.e., attitude, subjective norm, perceived behavioral control, and intention to gain muscle mass/lean body mass), and eating disorders (dieting, control, and bulimia behaviors). The results revealed satisfactory fit indices for both samples. Autonomous and controlled motivations for sport were positively directly and indirectly related to eating disorders in these athletes. This motivational mechanism needs more in-depth investigation, and motivational profiles might help distinguish athletes with and without eating disorders.