Background: Adolescents who experience the loss of a family member are at increased risk of adverse mental health outcomes, yet many face barriers or may be reluctant to access in-person or group-based support. mHealth (mobile health) interventions can help address these barriers by offering flexible, accessible, and low-threshold support. Objective: This study evaluated the short- and long-term mental health effects of Alba – Youth in Grief, a preventive self-management mobile app for bereaved adolescents. The primary outcome was symptoms of prolonged grief, while secondary outcomes included grief reactions, personal growth, and symptoms of posttraumatic stress and depression. User-reported helpfulness and negative experiences were also examined. Methods: In an unblinded randomized controlled trial (ClinicalTrials.gov NCT06093113), 126 adolescents aged 12‐19 years who had lost a parent or sibling were allocated to either the unguided Alba app (n=61) or an active control condition receiving unguided web-based psychoeducation (n=65). Online self-assessments were conducted at baseline and at 2, 6, and 12 months. Participants generally demonstrated high levels of distress at baseline, with 40% (50/126) reporting symptoms indicative of probable prolonged grief disorder according to ICD-11 (International Classification of Diseases, 11th Revision) diagnostic scoring rules. Mental health outcomes were analyzed using linear mixed models to examine changes over time between groups, while user experiences were examined using descriptive statistics and summative content analysis. Results: Intention-to-treat analyses showed moderate reductions in prolonged grief symptoms at 12 months among adolescents randomized to Alba compared with the control group, with no significant effects at the 2- and 6-month follow-ups. The app group also demonstrated greater reductions in grief reactions, posttraumatic stress symptoms, and depressive symptoms compared with controls, with the strongest effects observed at long-term follow-up. No effect on personal growth was demonstrated. Most participants reported the app as helpful, while a minority disclosed negative experiences such as sadness. Conclusions: Overall, the findings indicate that Alba may be beneficial in reducing mental health symptoms among bereaved adolescents and highlight its potential as a safe, acceptable, and scalable mHealth intervention.