The patient in this case was a 69-year-old male. While undergoing treatment for gastric cancer, he visited another hospital with a complaint of difficulty breathing and was hospitalized under a diagnosis of chronic obstructive pulmonary disease (COPD) exacerbation. He was transferred to our hospital for intensive examination and treatment. However, the COPD exacerbation findings were not observed and a contrast-enhanced computed tomography (CT) scan indicated no apparent thrombosis in the pulmonary arteries. A heart ultrasound examination indicated right-sided heart enlargement along with an increase in tumor markers. We suspected pulmonary tumor thrombotic microangiopathy (PTTM) and performed a right-sided heart catheterization. This revealed an elevated mean pulmonary arterial pressure and Class V in pulmonary wedge aspiration cytology. These findings were clinically diagnosed as PTTM associated with advanced gastric cancer. Respiratory failure improved through treatment with an anticancer agent and pulmonary vasodilators. He was discharged on hospital day 57. We herein report on a rare case of PTTM with successful recovery after an antemortem diagnosis, believing that the findings are valuable.