We report the case of a perforated duodenal ulcer and diffuse peritonitis associated with an incarcerated hiatal hernia. A 77-year-old woman with a 17-year history of rheumatoid arthritis treated with nonsteroidal anti-inflammatory drugs, who had also been receiving treatment for non-Hodgkin's lymphoma over 4 years, was referred to us for investigation of nausea and vomiting. An abdominal compute tomography (CT) scan showed an incarcerated hiatal hernia and free air in the hernia sac. Emergency laparotomy revealed an incarcerated hiatal hernia involving the stomach, transverse colon, and omentum. A perforated ulcer was also found in the posterior wall of the first portion of the duodenum. The combination of these disorders is thought to be rare in patients with a hiatal hernia and free air in its sac. As the reported mortality of perforated gastric ulcer associated with a hiatal hernia is high, early elective surgery should be performed in patients with a duodenal ulcer associated with a hiatal hernia.