Objective: To design a smartphone app and evaluate its effect on promoting mental well-being and awareness of anxious symptoms in adolescents. Materials and Methods: A pilot cluster randomized controlled trial was conducted with two secondary schools (390 students, mean age 13.1 years) randomized to the intervention and control groups. An app was designed to encourage "Sharing, Mind, and Enjoyment (SME)" between students and their parents (e.g., express gratitude), including interaction and game elements. The app was used daily over 1 month. The waitlist control group was offered the app after completing all assessments. The primary outcomes were changes in SME behaviors measured at 1 month and 3 months. Secondary outcomes included subjective happiness, well-being, personal health and happiness, family health, happiness and harmony, self-perceived knowledge, and understanding of anxious symptoms. Two focus groups of students and three individual in-depth interviews of community partners were conducted. Results: Seventeen of 152 students (11.2%) in the intervention group used the app together with parents (adherent subgroup) and 69 (45.4%) used it without involving parents. The intervention group did not show significant difference in the change of SME behaviors at 1- or 3-month follow-up compared with the control group. However, the intervention group reported greater increase in the awareness of anxious symptoms at follow-ups than the control group (d = 0.52 at 1 month and d = 0.43 at 3 month, both P < 0.001). Post hoc analysis showed a significantly greater increase in SME-related behaviors in the adherent subgroup than the control group at 3 months (d = 0.46, P = 0.04). The interviews found favorable changes in app users, but motivation to use the app was low in general. Both students and community partners suggested primary school students would be more receptive users. Conclusions: The app did not show effectiveness in increasing SME behaviors of students, but increased awareness of anxious symptoms. Further improvements and tests are warranted. Trial Registration: ClinicalTrials.gov NCT03361475.

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