This is a 12-month longitudinal intervention study in adults (18-65 years) with moderate-severe IBS (IBS-SSS ≥175) evaluating a personalized, patient-centered multidisciplinary treatment delivered in a Swedish tertiary care setting. The program includes an internet-based IBS school followed by four evidence-based modules (physician-led medical management/education, dietician-led dietary intervention, psychologist-led IBS-focused behavioral therapy, and physiotherapy) delivered in a sequence chosen by the participant, with symptom evaluation after each module. Outcomes are assessed before and after treatment, with the primary endpoint defined as treatment response (IBS-SSS reduction ≥50 points), and secondary endpoints covering symptom/psychological measures, visceral sensitivity and biological stress plus gut biomarkers, and multimodal brain imaging (structural MRI, rs-fMRI, task fMRI, and insula MRS).