A man in 60s underwent total gastrectomy for advanced gastric cancer with para-aortic lymph node metastases. Although postoperative chemotherapy decreased the metastases, he was subsequently admitted with progressive respiratory distress. Pulmonary hypertension and right-sided heart failure developed, and he died of sudden cardiopulmonary arrest 30 hours after admission. Autopsy revealed widespread tumor embolism, fibrocellular intimal proliferation, and thrombus formation in the small arteries, consistent with a diagnosis of pulmonary tumor thrombotic microangiopathy (PTTM) associated with gastric cancer. Although PTTM a rare clinicopathological entity that causes severe pulmonary hypertension, it should be considered as a differential diagnosis for acute dyspnea or pulmonary hypertension in patients with carcinoma, regardless of clinical improvement.