BACKGROUND AND OBJECTIVES: Rates of mental health disorders are rising among adolescents and young adults. Therefore, scalable methods for preventing psychopathology in these age groups are needed. As repetitive negative thinking (RNT) is a risk factor for depression and anxiety disorders, targeting RNT via smartphone app promises to be an effective, scalable strategy. The current three-arm, parallel group, randomized controlled trial tested whether a self-help app designed to reduce RNT decreased psychopathological symptoms and RNT in adolescents and young adults at risk for mental disorders. METHOD: A sample of 16-22-year-olds with elevated levels of RNT (N = 365) were randomly allocated to either use a one of two self-help apps designed to reduce RNT for 6 weeks or to a waitlist. The full RNT-focused intervention app encompassed a variety of RNT-reducing strategies, whereas the concreteness training app focused on one of these strategies, namely, concrete thinking. RESULTS: The apps did not decrease depressive symptoms, anxiety symptoms and RNT relative to the waitlist. However, exploratory analyses using a minimum dose criterion showed that participants who used the full-RNT-focused intervention app more often, reported greater baseline to follow-up decreases in depressive symptoms compared to waitlist. LIMITATIONS: Include decreased power due to slightly more dropout than expected and limited generalizability due to the mostly female and highly educated sample. CONCLUSIONS: RNT-focused prevention via a self-help app did not decrease depression and anxiety, presumably due to too little engagement with the app content provided.