BACKGROUND: Many patients with inflammatory bowel disease (IBD) suffer from psychological distress, sleep disturbances, fatigue, and a reduced quality of life-even during remission. Mindfulness-Based Cognitive Therapy (MBCT) has been effective in other populations, and therefore it may also benefit IBD patients. The primary objective was to evaluate the effectiveness of MBCT plus treatment as usual (TAU) in reducing psychological distress compared to TAU alone. METHODS: This multicenter randomized controlled trial study included IBD patients in remission, aged 16 and older, who experienced at least mild levels of psychological distress (Hospital Anxiety and Depression Scale >/= 11). Assessments were conducted at baseline, post-intervention (3 months), and at 6, 9, and 12 months after baseline. This trial was registered at ClinicalTrials.gov: NCT04646785. RESULTS: A total of 142 participants were allocated to MBCT + TAU (n = 70) or TAU alone (n = 72). Group-delivered MBCT significantly reduced psychological distress (d = -0.61) and improved well-being (d = 0.40) post-intervention, with consolidation of the effects over time. Exploratory objective sleep metrics (ie, electroencephalography) showed a reduction in total sleep time (d = 0.67) after MBCT with an increase in the proportion of deep sleep (d = 0.70). While flare occurrence showed no difference, fecal calprotectin levels reduced in the MBCT + TAU group over the follow-up period (d = -0.49). CONCLUSIONS: Mindfulness-Based Cognitive Therapy can be considered a valuable addition to the limited effective psychosocial interventions set for IBD patients, as it reduces psychological distress and improves well-being. This study also shows the possible impact of MBCT on biological processes, such as sleep and inflammation.