The aim of the present study was to use the self-determination model of exercise motives to examine the relationship between extrinsic and intrinsic motives for exercise and a number of measures of psychological well-being. Undergraduate students purporting to exercise regularly (N = 227; 102 men, 125 women) were split into 2 groups: those exercising for less than 6 months and those exercising for 6 months or more. The respondents were asked to complete measures of exercise motivation, self-esteem, psychological well-being, and stress. Among individuals exercising for less than 6 months, a number of extrinsic motivations for exercise were significantly related to poorer psychological well-being. Among individuals exercising for 6 months or more, a number of intrinsic motivations were significantly related to better psychological well-being. The present findings suggest that researchers can use self-determination theory to understand the relationship between exercise motivation and psychological well-being.