This study evaluated the acceptability and effectiveness of a relationship-focussed mobile phone application (WeClick) for improving depressive symptoms and other mental health outcomes in adolescents.A randomised controlled trial involving 193 youth (M age: 14.82, SD: 0.94, 86.5% female) from Australia was conducted. Youth were recruited via the Internet and randomly allocated to the intervention or a 4-week wait list control condition, stratified for age and gender. The primary outcome was change in depressive symptom scores measured using the Patient Health Questionnaire for Adolescents (PHQ-A) at baseline, 4-week post-test and 12-week follow-up. Secondary outcomes included anxiety, psychological distress, wellbeing, help-seeking intentions for mental health, social self-efficacy and social support. Participants in the intervention condition received access to the intervention for four weeks. Thematic analysis was utilised to identify and examine acceptability.The change in PHQ-A scores from baseline to 4-week post-test did not differ significantly (d = 0.26, p = .138) between the intervention (Mchange = -2.9, SD = 5.3) and wait list control conditions (Mchange = -1.7, SD = 4.3). However, significant between-group improvements were observed in wellbeing (d = 0.37, p = .023), help-seeking intentions (d = 0.36, p = .016) and professional help-seeking intentions for mental health problems (d = 0.36, p = .008). Increases in help-seeking intentions were sustained at follow-up in the intervention condition. No differential effects were found for generalised anxiety, separation anxiety, social self-efficacy or for any social support outcomes. Over 90% of participants indicated the app was enjoyable, interesting and easy to use. The app provided 'advice and direction' (n = 42; 46.15%), an 'opportunity for self-reflection' (n = 33; 36.3%) and 'normalised experiences' (n = 21; 23.1%).The WeClick app was found to be effective for improving wellbeing and help-seeking intentions for mental health in adolescents. A larger, adequately powered trial is now required to establish differential effects on depressive symptoms. This trial was registered with the Australian New Zealand Clinical Trials Registry (ANZCTR): ACTRN12618001982202.