A 56-year-old man with chief complaints of dry cough and dyspnea was admitted. He had severe hypoxemia, and his chest radiographs showed enhancement of pulmonary artery opacities with multiple defects on pulmonary blood flow scintigraphy. Enhanced computed tomography (CT) revealed swelling of the mediastinum and hilar lymph nodes, but no apparent thrombi in the pulmonary arteries was seen. A biopsy specimen of a left neck lymph node showed poorly differentiated adenocarcinoma, including signet-ring cell carcinoma components, but the origin was unclear. Despite receiving chemotherapy, his respiratory condition worsened, and he died 3 days after admission. Routine autopsy failed to clarify the tumor origin, but a detailed dissection of specimens confirmed early gastric cancer. Additionally, pathology of the pulmonary arteries was compatible with pulmonary tumor thrombotic microangiopathy (PTTM). PTTM is a rare condition characterized by the presence of diffuse thrombotic microthrombi and fibrocellular intimal proliferation in the pulmonary vasculature. Accompanied with early gastric cancer, this is an extremely rare but important case of PTTM.