Repetitive negative thinking (RNT) has been identified as a transdiagnostic process that is involved in various forms of psychopathology, including anxiety and depression. This randomized controlled trial compared a 5-week internet and app-based gratitude intervention (intervention group; IG) with adherence-focused guidance against a wait list control group (WLG) in reducing RNT in a sample with elevated RNT. Method: A total of 260 individuals were randomized to either the IG or the WLG. Data were collected at baseline (T1), within one week post intervention (T2), and at three (3-MFU) and six-months of follow-up (6-MFU; for IG only). The primary outcome was RNT. Secondary outcomes included other mental health outcomes and resilience factors. Results: Participants of the IG reported significantly less RNT at T2 (d = 0.61) and 3-MFU (d = 0.75) as compared to WLG. Improvements were sustained until 6-MFU. Significant, small to moderate effect sizes were identified for most secondary outcomes at T2 and 3-MFU. Furthermore, results of mediation analyses revealed that the gratitude intervention exerts its effect on anxiety and depression by reducing the risk factor of RNT, while the mediating role of resilience was less clear. Conclusions: The gratitude intervention investigated in this study was found to be effective in reducing RNT. Gratitude interventions might affect mental health by two parallel pathways: increasing resources and reducing risk factors. Reference number Ethics Committee of the University of Lueneburg: EB 201701-03-Lehr. Clinical trial registration number: DRKS00011862. The trial protocol can be assessed at: https://www.drks.de/.