Self-determination theory suggests that when psychological needs for autonomy, competence, and relatedness are met, participants experience more self-determined types of motivation and more positive outcomes. Limited research has examined this mediational role of self-determined motivation in adult physical activity participants, and very few studies have included assessments of relatedness. This study tested the hypothesis that self-determined motivation would mediate the relationship between psychological need fulfillment and affective and behavioral outcomes. Adult dragon boaters (N = 558) between the ages of 19 and 83 completed a questionnaire on motivational aspects of dragon boating. Competence, relatedness, and autonomy all significantly predicted self-determined motivation, but self-determined motivation only partially mediated their relationship with positive and negative affect. These findings demonstrate the importance of all three needs in adult activity motivation and suggest that the relationships between needs, self-determination, and outcomes may be complex.