Background: The prevalence of irritable bowel syndrome (IBS) is relatively high, but up to now, no population based study in Iran has used the ROME III criteria. The aim of the present study was to determine the prevalence of IBS by using the ROME III criteria in the adult population of Iran. Methods: A face to face survey was conducted in a large area of the Tehran province. IBS was diagnosed by using a validated questionnaire based on the ROME III criteria. Results: The study population comprised 18,180 participants, with a female to male ratio of 1. 15.3% of participants complained of gastrointestinal (GI) symptoms, while the prevalence of IBS was estimated to be 1.1% (139 women, 59 men, p=0.000). IBS patients were more likely to be married, and older. The most common presenting symptoms of IBS were abdominal pain that was relieved by defecation (94%), change in fecal consistency (78%), and change in bowel frequency (70%). Constipation was predominant in 52% of IBS cases, diarrhea was predominant in 18%, and 8% experienced intermittent diarrhea and constipation. Conclusion: The prevalence of IBS is relatively low in the Iranian adult population according to the ROME III criteria. The most probable reasons are the specificity of ROME III criteria and the characteristic low prevalence of GI symptoms in the study population.