Summary: Objective: The aim of this study was to evaluate the beneficial effects of lifestyle modification and dietary intervention on the severity of symptoms, quality of life, anxiety and depression status in patients with irritable bowel syndrome. Methods: In this clinical interventional trial study, patients aged 15 to 65 years with IBS (based on the ROME IV criteria) who were referred to the gastrointestinal clinic of Taleghani hospital, Tehran, Iran between March 2020 and August 2020 were included. After initial evaluation, lifestyle modification intervention was performed. Statistical analysis was performed using SPSS software. Results: In the present study, 55 patients consisting in 27 men and 28 women with IBS were included. The patients‘ mean anxiety score (based on BAI) and mean depression score (based on BDI) were significantly reduced after lifestyle and diet modification intervention as compared to the baseline. The mean score of the patients‘ quality of life (based on IBS-QOL) was 68.36 ±24.90 before the intervention, which increased to 82.75 ±29.52 after the intervention, indicating a significant difference (P <0.001). Similarly, the mean score of severity of IBS symptoms (based on IBS-SSS) was 28.29 ±8.27 before the intervention, which was reduced to 20.87 ±8.47 after the intervention with a significant difference (P <0.001). Conclusion: Our findings suggest that lifestyle and dietary modification might be a feasible and effective treatment approach in patients with IBS, which has a positive and significant effect on amelioration of the severity of symptoms, improving the quality of life as well as alleviating symptoms of anxiety and depression. Key words: irritable bowel syndrome – anxiety – depression – quality of life – lifestyle modification