A perforated ulcer is a condition in which an untreated ulcer has burned through the mucosal wall in a segment of the gastrointestinal tract (e.g., the stomach or colon) allowing gastric contents to leak into the abdominal cavity.

A perforated ulcer can be grouped into a stercoral perforation which involves a number of different things that cause perforation of the intestine wall. The first symptom of a perforated peptic ulcer is usually sudden, severe, sharp pain in the abdomen. The pain is typically at its maximum immediately and persists. It is characteristically made worse by any movement, and greatly intensifies with coughing or sneezing.^{[citation needed]}

Causes include alcohol, smoking, consuming highly acidic foods and beverages (such as coffee), and nonsteroidal anti-inflammatory drugs (NSAIDs).

The ulcer is known initially as a peptic ulcer before the ulcer burns through the full thickness of the stomach or duodenal wall. A diagnosis is made by taking an erect abdominal/chest X-ray (seeking air under the diaphragm). This is in fact one of the very few occasions in modern times where surgery is undertaken to treat an ulcer. Many perforated ulcers have been attributed to the bacterium Helicobacter pylori.

Treatment generally requires immediate surgery.

Perforated peptic ulcer is a serious condition with an overall reported mortality of 5%–25%, rising to as high as 50% with age. The incidence of perforated ulcer is steadily declining, though there are still incidents where it occurs.