Multicomponent, school-based interventions are considered to be an effective method for improving fundamental movement skill (FMS) proficiency levels and physical activity (PA) among youth. This study aimed to evaluate if the youth-physical activity toward health intervention can improve FMS proficiency in a randomized controlled trial among adolescents. Participants were 482 adolescents aged 12-13 years from 20 schools. For an academic year, participants in 10 schools received the youth-physical activity toward health intervention. The remaining 10 schools received their regular weekly physical education lessons. Fifteen FMS were assessed using validated tools; their PA was assessed using accelerometers; their height, weight, and cardiorespiratory fitness were also recorded. Outcomes were assessed at baseline, postintervention, and 3 months later at retention. Multilevel analysis was performed using MLwiN 2.35 software. Significant intervention effects across time were observed for total object control (P < .0001; β = 2.04; 95% confidence interval, 1.16 to 2.92) and total locomotor (P < .0001; β = 2.13; 95% confidence interval, 1.44 to 2.82), with the greatest improvements evident for total FMS score (P < .0001; β = 4.04; 95% confidence interval, 2.39 to 5.69). The effects of the intervention were significant and positive for all children in the intervention group regardless of gender, weight status, or PA level (P = .03 to < .0001). Youth-physical activity toward health has the potential to improve FMS proficiency among adolescents regardless of gender, weight status, and activity levels.