We report an unusual case of chest wall tumor in a 27-year-old patient. A complete resection was accomplished, and the patient had an excellent postoperative course. Histologically, the mass was confirmed to be an ectopic synovial bursa cyst. Although rare, synovial cysts should be considered in any case of a fluctuating chest wall mass. We also discuss the etiology and diagnostic approach of cystic masses of the chest wall.