BACKGROUND Mental health problems in adolescents are prevalent while population-based interventions for improving mental wellbeing of adolescents are limited. OBJECTIVE To design a smartphone app and evaluate its effect on promoting mental wellbeing of adolescents and awareness of anxiety disorders. METHODS A pilot cluster randomised controlled trial was conducted with 2 secondary schools (390 students, mean age 13.1 years) randomised to the intervention and control groups. A smartphone app was designed for one month’s use to promote mental wellbeing through “Sharing, Mind and Enjoyment (SME)” related interactions between students and their parents (e.g., express gratitude to family in words or actions), with assistance of App designers in releasing SME tasks daily. The waitlist control group was offered the app after completing all assessments. The primary outcomes were SME behaviours measured at 3-month after baseline. Secondary outcomes included subjective happiness, wellbeing, personal health and happiness, family health, happiness and harmony, self-perceived knowledge and understanding of anxiety disorders. Two focus groups of students and three individual in-depth interviews of service providers were conducted. RESULTS In the intervention students, 11.2% used the app together with parents and 45.4% used it without involving parents. The intervention group did not show significant difference in the change of SME behaviours at 1-month or 3-month compared with the control group. The intervention group showed greater increase in the awareness of anxiety disorders at follow-ups than the control group (Cohen’s d=0.52 at 1-month and 0.43 at 3-month, both P CONCLUSIONS The app did not show effectiveness in increasing SME behaviours of students but was effective in increasing awareness of anxiety disorders. Further improvements and tests among younger children and their parents are warranted. CLINICALTRIAL ClinicalTrials.gov NCT03361475