A primary pulmonary artery sarcoma is a rare malignant tumor derived from the intimal layer of the pulmonary artery. Its clinical presentation can lead to a misdiagnosis of more common diseases such as thromboembolic disease. It is known to have a very poor prognosis. Therefore, the correct diagnosis of a primary pulmonary artery sarcoma is difficult and often delayed. We experienced a case of primary pulmonary artery sarcoma mimicking a pulmonary thromboembolism. The patient was admitted as a result of progressive dyspnea and coughing. The lung perfusion scan showed a large perfusion defect involving almost the entire right lung and suspicious small perfusion defects in the left upper lobe. Magnetic resonance imaging of the chest showed an enhancing nodule within the thrombus in the right pulmonary artery. The mass was removed completely by surgery, but the patient died as a result of shock.