BACKGROUND: Irritable bowel syndrome (IBS) is one of the world's most common gastrointestinal (GI) disorders, and current treatments do not meet patients' demands. This study aimed to investigate melatonin's therapeutic effects on IBS score, GI symptoms, quality of life, and sleep parameters in both groups of IBS patients with and without sleep disorders. METHODS: In this randomized double-blinded placebo-controlled trial study, 136 patients with a diagnosis of IBS based on ROME IV criteria were enrolled and then divided into two groups respecting having sleep disorders or not. Patients of each group were randomized in a 1:1 ratio to receive melatonin 6 mg daily (3 mg fasting and 3 mg at bedtime) for 2 months (8 weeks). Blocked randomization was used in this process. All patients were evaluated both at the beginning and the end of the trial regarding IBS score, GI symptoms, quality of life, and sleep parameters through valid questionnaires. RESULTS: In both groups of patients with and without sleep disorders, a significant improvement was observed in IBS score and GI symptoms, including the severity and the frequency of abdominal pain, the severity of abdominal bloating, satisfaction with bowel habits, disease's impact on patient's life, and stool consistency; however, there was no significant improvement in the frequency of defecations per week. In patients with sleep disorders, significant improvement in sleep parameters, including subjective sleep quality, sleep latency, sleep duration, sleep efficiency, and daytime dysfunction, was observed, while in patients without sleep disorders, there was no significant improvement in sleep parameters. In addition, quality-of-life improvement was observed in a significant number of melatonin recipients compared to placebo in both groups of patients. CONCLUSION: Melatonin can be considered an effective treatment for improving IBS score, GI symptoms, and quality of life in IBS patients with and without sleep disorders. It is also effective to improve sleep parameters in IBS patients with sleep disorders. TRIAL REGISTRATION: This study has been registered to the Iranian Registry of Clinical Trials (IRCT) with the approval number IRCT20220104053626N2 on the date of 13/02/2022.