Background and Objective: Irritable bowel syndrome (IBS) is one of the most common gastrointestinal disorders that reduce the quality of life of patients. The objective of this study was to evaluate the effectiveness of group therapy based on acceptance and commitment to disease severity, quality of life, and psychological well-being of patients with IBS. Materials and Methods: The research method was quasi-experimental with a pre-test-post-test design and 4-month follow-up. The statistical population included 160 patients with IBS referred to a subspecialty gastroenterology clinic in Rasht from September 2019 to February 2020. Using purposeful sampling method, 60 patients were selected based on the diagnostic criteria of Rome III and the opinion of the treating physician. They were randomly assigned to two groups of experimental and control, each including 30 people. Data collection tools were Rome III diagnostic criterion, Irritable Bowel Syndrome Symptoms Severity Questionnaire, Quality of Life Short Scale, and Psychological Well-Being Scale. Also, the acceptance and commitment group therapy intervention was performed in 8 sessions of 90 minutes (one session per week) according to Hayes and Strosahl instructions. Data were analyzed using a univariate analysis of covariance by SPSS software version 22. Results: The mean (SD) age of participants in the experimental group was 38. 5 (7. 7) and in the control group was 39. 6 (8. 1). The results showed that the mean (SD) severity of symptoms in the experimental group was 183. 4 (4. 1) in the pre-test, which decreased to 171. 4 (1. 4) in post-test and 171. 3 (1. 5) in follow-up (P<0. 001). However, the mean (SD) of the control group in the pre-test (1. 8) was 179. 8, which was 178. 8 (1. 6) in the post-test and 178. 6 (1. 4) in follow-up, which was not statistically significant. Also, the mean (SD) of quality of life in the experimental group was 78 (1. 5) in the pre-test, which increased to 87. 1 (2. 6) in post-test and 86. 5 (2. 2) in follow-up. However, the mean (SD) score of the control group in the pre-test (1. 7) was 77. 7, which was 78. 7 (1. 7) in post-test and 78. 5 (1. 6) in follow-up, which was not statistically significant. The mean (SD) of psychological well-being in the experimental group was 55. 7 (2. 2), which increased to 69. 6 (2. 3) in post-test and 68. 4 (2. 1) in follow-up (P<0. 001). However, the mean (SD) score of the control group in the pre-test was 57. 2 (1. 7) which was 58. 8 (2. 1) in post-test and 58. 5 (1. 8) in follow-up, which was not statistically significant. Conclusion: The study showed that acceptance and commitment group therapy was effective in reducing disease severity, increasing psychological well-being, and improving the quality of life in patients with irritable bowel syndrome. Therefore, this therapeutic approach can be used in the physical and mental health of patients with irritable bowel syndrome.