Osteosarcomas generally arise in appendicular skeletons, but rarely in the ribs. We described Tc-MDP bone scan findings from a 23-year-old man with right back pain. The images demonstrated elevated activity in the region overlapping the posterior 8th to 10th ribs and in the L9-10 vertebral bodies. CT showed an 8.8 × 8.3 cm mildly peripherally calcified mass arising from the right 10th rib involving the 9th and 10th thoracic vertebrae. Pathological examination confirmed primary osteosarcoma of the rib.