Background: The goals of participation youth sports are primarily concerned with the facilitation of positive youth development as opposed to outright success. Consequently, there are strong theoretical and empirical links between sports coaching and athlete development. Transformational leadership behaviours, in particular, have been theoretically linked to positive developmental outcomes within a youth sport context, while the coach-athlete relationship is a key tool used by coaches who aim to teach life skills to young athletes. Outright team success has also been shown to correlate with important developmental variables such as a mastery climate and athlete perception of youth sports coaches. Aims: The purpose of this study was to test the relationship between coach transformational leadership behaviours, the perceived quality of the coach-athlete relationship, team success, and the positive developmental experiences of adolescent soccer players. Method: Cross-sectional data were taken from 455 adolescent athletes aged between 11 and 18 years. Each participant was competing in a local soccer competition that is classified as a participation sport. Thus, the theoretical focus is on developmental and skill gains. Each participant completed the Differentiated Transformational Leadership Inventory for Youth Sport, the Coach-Athlete Relationship Questionnaire, and the Youth Experience Survey for Sport. Team success was measured by the total number of competition points accumulated during the season. Findings: The results show that coach transformational leadership behaviour and the coach-athlete relationship have a moderate positive correlation with developmental experiences. Team success has no relationship with overall developmental experiences. The best predictor of developmental experiences is a combination of coach transformational leadership behaviour and the quality of the coach-athlete relationship. The most influential leadership behaviours are individual consideration, intellectual stimulation, and appropriate role modelling. Discussion: Transformational leadership and the quality of the coach-athlete relationship may work synergistically to influence positive athlete outcomes within youth sports. Importantly, coaches who practise within the youth sport context are able to facilitate positive developmental outcomes from both team success and team failure by taking advantage of naturally occurring teachable moments. The best way to take advantage of these may be to engage in intellectual stimulation, individual consideration, and positive role modelling, in addition to facilitating positive, developmentally appropriate coach-athlete relationships. Future coach education programmes should incorporate relevant interpersonal and intrapersonal skills that allow youth sports coaches to engage in these behaviours. Longitudinal work is needed in order to make causal inferences between transformational leadership behaviour, the quality of the coach-athlete relationship, and positive youth development through sports.