A 78-year-old woman presented with a 2-month history of abdominal pain, fever, nausea, and vomiting. After admission, abdominal computed tomography showed multiple tumors in the abdominal cavity in close contact with the small bowel loops. She underwent exploratory laparotomy, with intraoperative findings of multiple tumors measuring up to 12 cm over the abdominal and pelvic cavities and tumor adhesion to the small intestine, colon, and omentum. Due to the difficulties associated with debulking surgery, two tumor nodules were removed from the omentum for pathologic examination, including immunohistochemistry, which led to the diagnosis of pathological stage III primary peritoneal mesothelioma (MPM). The patient died 1 month after surgery due to aspiration pneumonia with respiratory failure. MPM has a poor prognosis; it is treated with a combination of systemic chemotherapy, intraperitoneal chemotherapy, cytoreductive surgery, and whole-abdominal irradiation. The median survival time for this type of tumor is consistently less than 1 year.