OBJECTIVES: To assess the effectiveness of a positive youth development (PYD)-based sports mentorship program on the physical and mental well-being of adolescents recruited in a community setting. METHODS: This is a randomized controlled trial in which we recruited students from 12 secondary schools in Hong Kong, China. Participants were randomly assigned in a 1:1 ratio to an intervention or a control arm after stratification for school from October 2013 to June 2014. Participants were not blinded to allocation because of the nature of the intervention. Students in the intervention arm received an after-school, PYD-based sports mentorship for 18 weeks. Each weekly session lasted 90 minutes. Students in the control arm received exclusive access to a health education Web site. RESULTS: Six hundred and sixty-four students (mean age 12.3 years [SD 0.76]; 386 girls [58.1%]) completed baseline and postintervention assessments. The intervention improved students’ mental well-being (Cohen’s d, 0.25; 95% confidence interval [CI], 0.10 to 0.40; P = .001), self-efficacy (Cohen’s d, 0.22; 95% CI, 0.07 to 0.37; P = .01), resilience (Cohen’s d, 0.19; 95% CI, 0.03 to 0.34; P = .02), physical fitness (flexibility [Cohen’s d, 0.28; 95% CI, 0.13 to 0.43; P = .02], lower limb muscle strength [Cohen’s d, 0.18; 95% CI, 0.03 to 0.33; P = .03], and dynamic balance [Cohen’s d, 0.21; 95% CI, 0.06 to 0.37; P = .01]), and physical activity levels (Cohen’s d, 0.39; 95% CI, 0.24 to 0.55; P < .0001). The intervention did not significantly improve physical well-being (Cohen’s d, −0.01; 95% CI, −0.17 to 0.14; P = .86), BMI z scores (Cohen’s d, −0.03; 95% CI, −0.18 to 0.12; P = .69), body fat proportion (Cohen’s d, −0.15; 95% CI, −0.31 to 0.00; P = .051), and social connectedness (Cohen’s d, −0.03; 95% CI, −0.18 to 0.12; P = .72). CONCLUSIONS: A PYD-based sports mentorship intervention improved healthy adolescents’ mental well-being, psychological assets, physical fitness, and physical activity levels.