Diverticulitis, also called colonic diverticulitis, is a gastrointestinal disease characterized by inflammation of abnormal pouches—diverticula—that can develop in the wall of the large intestine. Symptoms typically include lower abdominal pain of sudden onset, but the onset may also occur over a few days. There may also be nausea, diarrhea or constipation. Fever or blood in the stool suggests a complication. People may experience a single attack, repeated attacks, or ongoing "smoldering" diverticulitis.

The causes of diverticulitis are unclear. Risk factors may include obesity, lack of exercise, smoking, a family history of the disease, and use of nonsteroidal anti-inflammatory drugs (NSAIDs). The role of a low fiber diet as a risk factor is unclear. Having pouches in the large intestine that are not inflamed is known as diverticulosis. Inflammation occurs in 10% and 25% at some point in time and is due to a bacterial infection. Diagnosis is typically by CT scan. However, blood tests, colonoscopy, or a lower gastrointestinal series may also be supportive. The differential diagnoses include irritable bowel syndrome.

Preventive measures include altering risk factors such as obesity, physical inactivity, and smoking. Mesalazine and rifaximin appear useful for preventing attacks in those with diverticulosis. Avoiding nuts and seeds as a preventive measure is no longer recommended since there is no evidence that these play a role in initiating inflammation in the diverticula. For mild diverticulitis, antibiotics by mouth and a liquid diet are recommended. For severe cases, intravenous antibiotics, hospital admission, and complete bowel rest may be recommended. Probiotics are of unclear value. Complications such as abscess formation, fistula formation, and perforation of the colon may require surgery.

The disease is common in the Western world and uncommon in Africa and Asia. In the Western world about 35% of people have diverticulosis while it affects less than 1% of those in rural Africa, and 4–15% of those may go on to develop diverticulitis. In North America and Europe the abdominal pain is usually on the left lower side (sigmoid colon), while in Asia it is usually on the right (ascending colon). The disease becomes more frequent with age, ranging from 5% for those under 40 years of age to 50% over the age of 60. It has also become more common in all parts of the world. In 2003 in Europe, it resulted in approximately 13,000 deaths. It is the most frequent anatomic disease of the colon. Costs associated with diverticular disease were around US$2.4 billion a year in the United States in 2013.

Diverticulitis typically presents with lower quadrant abdominal pain of a sudden onset. Patients commonly have elevated C-reactive protein and a high white blood cell count. In Asia it is usually on the right (ascending colon), while in North America and Europe, the abdominal pain is usually on the left lower side (sigmoid colon). There may also be fever, nausea, diarrhea or constipation, and blood in the stool. Diverticulosis is associated with more frequent bowel movements, contrary to the widespread belief that patients with diverticulosis are constipated.

In complicated diverticulitis, an inflamed diverticulum can rupture, allowing bacteria to subsequently infect externally from the colon. If the infection spreads to the lining of the abdominal cavity (the peritoneum), peritonitis results. Sometimes, inflamed diverticula can cause narrowing of the bowel, leading to an obstruction. In some cases, the affected part of the colon adheres to the bladder or other organs in the pelvic cavity, causing a fistula, or creating an abnormal connection between an organ and adjacent structure or another organ (in the case of diverticulitis, the colon, and an adjacent organ).

Related pathologies may include:

The causes of diverticulitis are poorly understood. Formation of diverticula is regarded as likely due to interactions of age, diet, colonic microbiota, genetic factors, colonic motility, and changes in colonic structure.