GOALS: To test the efficacy of a self-help cognitive behavioral therapy (CBT) for irritable bowel syndrome (IBS) app compared with an active control app. BACKGROUND: IBS is a disorder of gut-brain interaction that can result in significant distress, disability, and psychiatric co-morbidity. CBT is an effective treatment for IBS. Self-help CBT apps can increase accessibility but should be tested against active controls. METHODS: This randomized controlled trial (NCT04665271) compared a CBT for IBS self-help app (Zemedy) to an active education, lifestyle management, relaxation, and mindfulness control app. A total of 453 individuals were screened in and offered allocation to treatment. Participants who actually downloaded their assigned app (N=267) were evenly split between the CBT app (N=136) and the active control (N=131). Follow-up data (CBT N=74, control N=82) were collected immediately post-treatment, at which point the control group was offered crossover to CBT. Follow-up data were collected at 3 (N=65) and 6 (N=32) months. Primary outcomes included IBS symptom severity and IBS quality of life. Secondary outcomes included catastrophizing, visceral anxiety, fear of food, and depression. RESULTS: At post-treatment, the CBT group improved significantly across all outcomes. The control group also improved on all outcomes except fear of food. In the intent-to-treat analysis the CBT group improved significantly more than the control group on both primary and secondary outcomes except depression. Gains were maintained at 3 and 6 months, although attrition was considerable. CONCLUSION: Self-help CBT for IBS may be effective and can be delivered successfully through apps, although we did not have data on engagement. Psychoeducation about symptom management strategies, relaxation, and mindfulness are active treatment components, but CBT is better at addressing underlying maintaining factors like catastrophizing, visceral anxiety, and fear of food.