A man in his 50s presented to our hospital for anemia and was diagnosed with advanced gastric cancer with para-aortic lymph node metastases. He underwent gastrojejunostomy and received postoperative chemotherapy. Despite stable disease with chemotherapy, he complained of cough and respiratory discomfort and was subsequently admitted with progressive respiratory distress. Pulmonary hypertension and right-sided heart failure developed, and he died of sudden cardiopulmonary arrest 9 days after admission. An autopsy revealed widespread tumor metastasis, and he was diagnosed with pulmonary tumor thrombotic microangiopathy(PTTM)associated with gastric cancer. Although PTTM is a rare clinicopathological entity that causes severe pulmonary hypertension, it should be considered in the differential diagnosis of acute dyspnea in patients with carcinoma, regardless of clinical improvement.