BACKGROUND: Irritable bowel syndrome (IBS) is a common disease that affects the quality of life (QOL) and social functioning of sufferers. Visceral anxiety is currently considered a key factor in the onset and exacerbation of IBS, and cognitive-behavioural therapy (CBT) targeting visceral anxiety is thought to be effective. However, access to CBT is limited due to the lack of trained therapists, the substantial time required for therapy and the associated costs. Group CBT (GCBT) may solve some of these problems. We have therefore planned this trial to examine the efficacy of GCBT for IBS. METHODS: The trial is a two-armed, parallel group, open label, stratified block randomized superiority trial. The study group will consist of 112 participants (aged 18-75 years) with IBS (Rome-III or IV criteria). Participants will be randomly allocated 1:1 to (i) the intervention group: ten-week GCBT plus treatment as usual (TAU) or (ii) the control group: waiting list (WL) plus TAU. The co-primary outcomes are the change in IBS severity or disease-specific quality of life from baseline to week 13 which is 1 month after the end of treatment. The efficacy of GCBT for IBS will be examined through mixed-effects repeated-measures analysis. DISCUSSION: GCBT, if found effective, can address the issues of the shortage of therapists as well as the time required and the costs associated with individual CBT. Clinically, the findings will help make effective CBT programmes accessible to a large number of distressed IBS patients at lower costs. Theoretically, the results will clarify the relationship between IBS and psychological stress and will help elucidate the underlying mechanisms of IBS. TRIAL REGISTRATION: UMIN, CTR-UMIN000031710. Registered on March 13, 2018.