Peritonitis is inflammation of the membranes of the abdominal wall and organs. Peritonitis is a life-threatening emergency that needs prompt medical treatment.

The abdominal organs, such as the stomach and liver, are wrapped in a thin, tough membrane called the visceral peritoneum. The abdominal walls are similarly lined (parietal peritoneum). A protective layer of fat contained in a membrane (the omentum) sits between the organs and the abdominal wall. Lubricating fluid allows all these membranes to slide smoothly over each other. The main function of the peritoneum is to permit free movement of the internal organs during digestion. Peritonitis is inflammation of the peritoneum caused by bacterial infection.

Peritonitis may be classed as spontaneous peritonitis or secondary peritonitis. Treatment options include hospitalisation, antibiotics and surgery.