A 63-year-old man was admitted to our hospital due to severe hypoxemia. D-dimer levels were elevated and echocardiography suggested pulmonary hypertension. Although a pulmonary thromboembolism (PTE) was suspected, contrast-enhanced computed tomography showed no apparent thrombi in the pulmonary arteries but prostate cancer with multiple bone metastases. Pulmonary perfusion scintigraphy showed multiple nonsegmental defects. We suspected pulmonary tumor thrombotic microangiopathy (PTTM) and performed a right heart catheterization. Pulmonary hypertension was confirmed, and pulmonary wedge aspiration cytology revealed malignant cells. A clinical diagnosis of PTTM was obtained. Respiratory failure improved after pulmonary vasodilators for pulmonary hypertension and hormone therapy for prostate cancer were started. Clinician should be aware of the possibility of PTTM in cancer patients who exhibit respiratory failure accompanied by pulmonary hypertension without an obvious PTE. PTTM can be a fatal complication, but prompt diagnosis and anti-cancer therapy with a pulmonary vasodilator may improve the prognosis.