The condition of a 52-year-old man was diagnosed with unresectable retroperitoneal extraskeletal osteosarcoma. SPECT/CT demonstrated a retroperitoneal extraskeletal osteosarcoma adjacent to the right upper ureter with large calcifications and increased Tc-MDP accumulation. Multiple hepatic capsular, renal fascial, and peritoneal metastases were noted. Four months later, multiple peritoneal metastases were noted at the second-look surgery. A histopathologic examination confirmed osteosarcoma.