This case is of a 56-year-old woman with a past history of primary peritoneal cancer after surgery and chemotherapies. She was admitted to our hospital with dyspnea. A chest enhanced CT showed no abnormality other than that of pulmonary artery enlargement. Pulmonary hypertension was diagnosed on the basis of an estimated right ventricular systolic pressure of 72 mmHg by cardiac ultrasonography. After admission, she was treated for pulmonary hypertension. However, respiratory failure rapidly worsened, and she died after 48 h of hospitalization. We performed an autopsy in which prominent fibrocellular intimal proliferation of the small pulmonary arteries and arterioles was identified. Tumor emboli were also observed in many places within the arteries and arterioles. Because the tumor cells were the same as the peritoneal cancer cells of the surgery specimen, we diagnosed pulmonary tumor thrombotic microangiopathy caused by peritoneal cancer.