Peritonitis is infection (or inflammation) of the peritoneum, which is a two-layered membrane covering both the surfaces of the organs that lie in the abdominal cavity and the inner surface of the abdominal cavity itself. It is frequently life-threatening and acute peritonitis is a medical emergency. Outlook for untreated peritonitis is very poor.

A major cause of bacterial peritonitis is internal perforation of the gastrointestinal tract, contaminating the abdominal cavity with bacteria from gastric contents. This may result as a complication of an intestinal foreign body or a ruptured appendix, a possible consequence of untreated acute appendicitis. The possibility of peritonitis is the reason why acute appendicitis warrants fast treatment (generally, appendicectomy), and other possible causes equally require laparotomy for inspection and treatment.