Background: Depressive and anxiety symptoms are prevailing among transitional aged youth (TAY) putting this population at high risk for lifelong mental illness as these disorders track through adulthood. Heavy social media use (SMU) is prevalent among TAY, which is concerning given it is associated with emotional distress in the form of depression and anxiety, with evidence that unfavourable social comparisons may be a primary mechanism. However, most research is cross-sectional in nature, highlighting the need for experimental studies to provide a greater understanding of how SMU impacts mental health and social comparisons. Accordingly, the primary aim of this study was to examine whether reducing SMU to 1 hour/day reduced depressive symptoms in distressed TAY. Secondary objectives included the examination of intervention effects on anxiety symptoms, and whether social comparison mediated the relationship between SMU reduction and reduced depressive and anxiety symptoms. Exploratory objectives included examining the intervention effects on individuals with greater/lesser emotional distress at baseline. Design and Methods: A randomized controlled trial was used to assign 218 (77% female) TAY (aged 17-25 years) to either the intervention group (n=116) or the control group (n=102). The intervention group was instructed to reduce smartphone-based SMU to a maximum of 1hr/day while the control group had no SMU restrictions. Study duration included 1-week baseline period followed by a 3-weeks intervention period; and mental health outcome measurements were taken at baseline and 4-weeks. Depressive symptoms were measured using the Center for Epidemiologic Studies Depression Scale (CESD-10), and anxiety symptoms were measured using the Generalized Anxiety Disorder scale (GAD-7), while social comparison was measured using the Technology-Based Social Comparison and Feedback-Seeking subscale (SCFS), which is a subscale of the Motivations for Electronic Interaction Scale (MEIS). Results: Analysis of variance (ANOVA) revealed the intervention was successful in limiting SMU, evidenced by a significant group x time interaction showing a greater reduction of SMU in the intervention group compared to the control group (p <.001, parital ηₚ²=.286). Simple effects showed that intervention participants spent less time in SMU than controls at week two (79.8min (45.6) vs 186.8min (98.21), p < .001), week three (75.97min (53.78) vs 189.67min (84.38), p < .001), and week four (76.01min (57.48) vs 188.8min (94.94), p <.001). There was good adherence to submitting screenshots (94%) and this did not differ by group at baseline (p = 0.474) or during the 3-week intervention period (p = 0.834). ANOVA also revealed a significant group x time interaction showing a greater reduction for the intervention group compared to the control group at week 4 in depressive symptoms (p= .019, partial ηₚ²= .025) and anxiety symptoms (p = .015, partial ηₚ²= .027), but groups did not differ on changes in social comparison (p= .593, partial ηₚ²= .001). Three-way interactions showed that relative to controls, SMU reduction produced a greater reduction in anxiety symptoms for those with high baseline scores (p = .008, partial ηₚ²= .032) compared to those with low baseline scores, whereas this three-way interaction was not significant for depressive symptoms (p = .438, ηₚ²=.003). Conclusion: Our data suggest that reducing SMU to approximately 1hr/day may be a feasible, inexpensive and effective therapeutic component that should be considered in the overall management of anxiety and depressive symptoms in TAY with emotional distress, a vulnerable population for lifelong mental illness.