Primary osteosarcomas are one of the most common malignant bone tumors principally affecting the long bones in children and adolescents. An unusual case of a primary osteoblastic osteosarcoma of the rib in a 42-year-old male is presented here. The patient underwent a wide excision of the tumor and chest wall reconstruction. Although clinically unsuspected in this unusual site, the classic microscopic feature of a ramifying osteoid matrix amidst the tumor cells was diagnostic of an osteosarcoma.