Objectives: To investigate the effectiveness of the CrossFitTM Teens resistance training program for improving mental health outcomes in adolescents, and to explore potential moderators and mediators. Design: Assessor-blinded randomized controlled trial. Methods: Ninety-six students (15.4 (0.5) years, 51.5% female) from one NSW secondary school, Australia 2013, were randomized into the 8-week CrossFitTM Teens intervention (n = 51) or control conditions (n = 45). Measures of mental health (psychological distress and self-esteem) were assessed using the Strength and Difficulties Questionnaire and Physical Self-Description Questionnaire. Hypothesized mediators were perceived body fat, strength and appearance; and general physical self-concept. Mediation was assessed using Preacher and Hayes’ multiple mediation regression SPSS macro. Hypothesized moderators (sex and baseline levels of psychological distress) were assessed using linear mixed models and Cohen’s d effect sizes were evaluated. Results: There were no significant intervention effects on mental health or potential mediators in the full study sample. Intervention participants categorized as ‘at risk’ of psychological distress demonstrated improvements in self-esteem (d = 1.35); perceived body fat (d = 1.05), perceived appearance (d = 0.95); physical self-concept (d = 1.96); and total difficulties score (d = 0.70). A medium-large positive effect on perceived body fat was also observed in boys. Conclusions: Participation in the CrossFitTM Teens resistance training program did not improve mental health outcomes in the full study sample. However, the results from this study provides preliminary evidence for improving mental health in adolescents ‘at risk’ of developing psychological disorders.