We report the case of a heterotopic ossification in the subscapularis muscle. A 30-year-old woman was diagnosed as a heterotopic ossification in the right subscapularis muscle. X-rays, a CT scan and a MRI showed bone-like lesions in the muscle. The location of the ossification was both inferior and anterior to the joint. The mass nearly bridged between the proximal humerus and the inferior portion of the coracoid process. Symptoms did not respond to rest, NSAIDs for 3 months and to stretching exercises treatment for 6 months. Excision and biopsy was performed through deltopectoral approach. Disodium Etidronate was administered during the postoperative period. She regained normal range of motion postoperatively and improved in the shoulder pain after performing strengthening exercises with Therabands. We think that a cause of limitation of the shoulder was impingement between the mass and the coracoid process, and that another cause was adhesion between the mass and the joint capsule. We conclude that excision and postoperative rehabilitation exercises are good methods for a heterotopic ossification in the subscapularis muscle for those that are nonresponsive to nonoperative treatments.