The patient is a 76-year-old man. His chief complaint of chest pain led to a diagnosis of pericardial effusion of unknown cause, and pericardial drainage was performed. On the 30th day, chest pain appeared again. Echocardiography revealed a pericardial fluid reaccumulation and a substantial mass in the pericardial space. Surgical drainage was performed to find the cause. A hematoma/mass was present on the epicardium. The pericardial sac was filled with hematoma. The hematoma was removed, but part of the mass infiltrated close to the anterior descending branch of the left coronary artery, and removal of that part was abandoned. The intrapericardial hematoma and epicardium were submitted to pathology leading to the diagnosis of synovial sarcoma. The patient was discharged home 14 days after surgery.