Irritable bowel syndrome (IBS) is a "disorder of gut-brain interaction" characterized by a group of symptoms that commonly include abdominal pain and or abdominal bloating and changes in the consistency of bowel movements. These symptoms may occur over a long time, sometimes for years. IBS can negatively affect quality of life and may result in missed school or work or reduced productivity at work. Disorders such as anxiety, major depression, and chronic fatigue syndrome are common among people with IBS.

The causes of IBS may well be multi-factorial. Theories include combinations of "gut–brain axis" problems, alterations in gut motility, visceral hypersensitivity, infections including small intestinal bacterial overgrowth, neurotransmitters, genetic factors, and food sensitivity. Onset may be triggered by an intestinal infection ("post-infectious irritable bowel syndrome") or a stressful life event.

Diagnosis is based on symptoms in the absence of worrisome features and once other potential conditions have been ruled out. Worrisome or "alarm" features include onset at greater than 50 years of age, weight loss, blood in the stool, or a family history of inflammatory bowel disease. Other conditions that may present similarly include celiac disease, microscopic colitis, inflammatory bowel disease, bile acid malabsorption, and colon cancer.

Treatment of IBS is carried out to improve symptoms and can be very effective. This may include dietary changes, medication, probiotics, and counseling. Dietary measures include increasing soluble fiber intake, or a diet low in fermentable oligosaccharides, disaccharides, monosaccharides, and polyols (FODMAPs). The "low FODMAP" diet is meant for short to medium term use and is not intended as a life-long therapy. The medication loperamide may be used to help with diarrhea while laxatives may be used to help with constipation. There is strong clinical-trial evidence for the use of antidepressants, often in lower doses than that used for depression or anxiety, even in patients without comorbid mood disorder. Tricyclic antidepressants such as amitriptyline or nortriptyline and medications from the selective serotonin reuptake inhibitor (SSRI) group may improve overall symptoms and reduce pain. Patient education and a good doctor–patient relationship are an important part of care.

About 10–15% of people in the developed world are believed to be affected by IBS. The prevalence varies according to country (from 1.1% to 45.0%) and criteria used to define IBS; however pooling the results of multiple studies gives an estimate of 11.2%. It is more common in South America and less common in Southeast Asia. In the Western world it is twice as common in women as men and typically occurs before age 45. However, women in East Asia are not more likely than their male counterparts to have IBS, indicating much lower rates among East Asian women. There is likewise evidence that men from South America, South Asia and Africa are just as likely to have IBS as women in those regions, if not more so. The condition appears to become less common with age. IBS does not affect life expectancy or lead to other serious diseases. The first description of the condition was in 1820, while the current term irritable bowel syndrome came into use in 1944.