Irritable bowel syndrome (IBS) is a common functional gastrointestinal disorder that substantially affects quality of life. Dietary therapy is considered one of the primary treatment approaches; however, the optimal sequence of dietary interventions remains unclear. This randomized crossover trial was designed to compare the effects of a low-FODMAP diet and a lactose-free, gluten-free diet on gastrointestinal symptoms, psychological status, anthropometric measurements, and biochemical parameters in adults with IBS. Participants were randomly assigned to one of two dietary sequences, with each intervention lasting four weeks and separated by a one-week washout period. Outcomes were assessed using validated symptom and psychological questionnaires together with anthropometric and biochemical measurements before and after each intervention period.