Introduction: The primary objective was to evaluate the impact of the ‘Switch-off 4 Healthy Minds’ (S4HM) intervention on recreational screen-time in adolescents. Methods: Cluster randomized controlled trial with study measures at baseline and 6-months (post-intervention). Eligible participants reported exceeding recreational screen-time recommendations (i.e., > 2 h/day). In total, 322 adolescents (mean age = 14.4 +/- 0.6 years) from eight secondary schools in New South Wales, Australia were recruited. The S4HM intervention was guided by Self-Determination Theory and included: an interactive seminar, eHealth messaging, a behavioral contract and parental newsletters. The primary outcome was recreational screen-time. Secondary outcomes included mental health (i.e., well-being, psychological distress, self-perceptions), objectively measured physical activity, and body mass index (BMI). Outcome analyses were conducted using linear mixed models and mediation was examined using a product-of-coefficients test. Results: At post-intervention, significant reductions in screen-time were observed in both groups, with a greater reduction observed in the intervention group (-50 min/day versus -29 min, p < 0.05 for both). However, the adjusted difference in change between groups was not statistically significant (mean = -21.3 min/day, p = 0.255). There were no significant intervention effects for mental health outcomes, physical activity or BMI. Significant mediation effects for autonomous motivation were found. Conclusions: Participants in both the S4HM intervention and control groups significantly reduced their screen-time, with no group-by-time effects. Enhancing autonomous motivation might be a useful intervention target for trials aimed at reducing adolescents' recreational screen-time.