Background: High relapse rates following acute treatment for common mental health disorders support the importance of developing maintenance phase interventions. Internet-based interventions have been effective for a broad range of mental disorders, but less is known about their potential to enhance long-term outcomes of traditional face-to-face therapy. Therefore, the goal of the present study was to evaluate a transdiagnostic Internet-based maintenance treatment (TIMT) with the purpose of fostering long-term effects of inpatient psychotherapy. Method: In this pragmatic randomized controlled trial, a sample of 400 inpatients with affective, neurotic, and/or behavioral disorders was assigned to either 12 weeks of TIMT + treatment as usual (TAU) or to TAU-only following hospital discharge. TIMT consists of a self-management module, asynchronous patient-coach communication, online patient support group, and online-based monitoring of psychopathological symptoms. Self-ratings of psychopathological symptoms were conducted at the beginning of inpatient treatment (t1), at discharge from inpatient treatment/start of TIMT (t2), and at 3-month (t3) and 12-month follow-ups (t4). Results: The TIMT + TAU group was superior to the TAU-only group with regard to differences in change of general psychopathological symptom severity from discharge to 3-and 12-month follow-up. Moreover, participants of the TIMT + TAU group showed less frequent symptom deteriorations and were more often in remission/recovery than controls. Conclusion: TIMT effectively enhances long-term outcome of inpatient psychotherapy.