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INTRODUCTION: Anxiety disorders are prevalent among youth, yet access to effective treatments is limited. OBJECTIVE: This study investigates the efficacy of the SMILE app, a smartphone application that delivers cognitive behavioral therapy (CBT)-based interventions to youth aged 12-21 with severe anxiety symptoms. The SMILE app uses mobile sensing technology to provide personalized interventions, addressing individual behavioral patterns. METHODS: In this two-arm randomized controlled trial, 107 participants, aged 12-21 with severe anxiety symptoms, were assigned to the intervention group, using the SMILE app in combination with a mobile sensing app, or the control group, using only a mobile sensing app. The primary outcome was anxiety symptoms measured by the SCARED scale. Secondary outcomes included depression and insomnia. Assessments were conducted at baseline, post-intervention (4 weeks), and follow-up (12 weeks). RESULTS: Data analysis followed an intention-to-treat approach. Anxiety symptoms significantly decreased in the treatment group compared to the control group at 4 weeks (-0.48, SE = 0.17; p = 0.006, 95% CI: -0.82 to -0.14), and marginally at 12 weeks (b = -0.32, SE = 0.18; p = 0.07, 95% CI: -0.68 to 0.03). Depression symptoms also showed significant reductions (b = -0.26, SE = 0.11, p = 0.016, 95% CI: -0.48 to -0.05). No significant changes were observed for insomnia. CONCLUSIONS: The SMILE app shows efficacy and potential scalability as a solution for youth with severe anxiety. These findings suggest that leveraging mobile sensing to tailor intervention delivery may enhance the smartphone-based delivery of CBT for youth with severe anxiety symptoms. TRIAL REGISTRATION: Clinicaltrials.gov (NCT06748833).