A primary giant cell tumor (GCT) originating from the sternum is extremely rare. We report a case of a GCT originating from the sternum in a 45-year-old man who was referred to us for a mass in the anterior chest wall that had been growing slowly. Computed tomography revealed a soft tissue mass involving a large osteolytic and destructive lesion of the sternum body. Subtotal sternectomy and reconstruction with methylmethacrylate were performed. The tumor was 8.5 x 4.5 x 2.5cm, and the histopathological examination confirmed GCT. Radical wide resection of primary sternum tumors and reconstruction with an appropriately rigid prosthetic material are necessary to minimize local recurrence.