Giant cell tumors (GCTs) of bone are rare primary neoplasms that usually originate from the epiphysis of long bones. Even when the tumor occurs in a rib, it is usually in the posterior aspect. We report a case of a GCT originating from the anterior arc of the rib in a 22-year-old woman presenting with a progressively growing thoracic mass in the right anterior chest wall. Through this case the authors emphasize not only the rarity of a bone GCT but also its unusual costal localization.