AI-powered mental health apps have attracted growing interest as a low-cost way to expand care. Yet questions remain about their effectiveness, safety, and whether they may crowd out psychotherapy. We evaluate one such app in a randomized controlled trial among 1,964 Mexican women with mild to severe psychological distress. Over six months, app access improved mental health by 0.3 standard deviations with no evidence of harm, improved sleep quality, increased healthful behaviors, and reduced missed work, yielding considerably larger benefits than costs. Treated participants were also more likely to seek traditional psychotherapy, but this increase does not explain most of the mental health gains. App use was high in the first month but then declined, as is common in digital interventions. Despite this drop in use, treatment effects persisted. Participants continued to implement practices promoted by the app, suggesting that even short-term engagement can produce durable improvements through sustained behavioral change.