Ganglion cysts are most common on the dorsum of the hand or wrist, but they can occur in any part of the body. There have been few papers reporting ganglion cysts originating from the sternoclavicular joint, with most of these cases developing in children. A 76-year-old woman was referred to our department because of a painless mass over the right sternoclavicular joint. The mass was excised along with the portion of the sternoclavicular joint capsule surrounding the stalk. Histopathologic examination showed the cyst wall to be composed of compressed collagen fibers without evidence of an epithelial or synovial lining, which was consistent with ganglion cyst. To our knowledge, this is the first report of such a cyst in an adult. We consider this to be a useful report for surgeons that treat mass lesions occurring in almost any part of the body surface.