We report a case of primary osteosarcoma of the breast, which is a rare histological type of all breast tumors. A 58-year-old woman had noticed a right breast mass long before presenting to our hospital because it had gradually increased in size. The mass was bony-hard, 90 x 70 mm in size, and was located mainly in the upper outer quadrant of the left breast. Mammography demonstrated a round radiopaque mass with a shaggy outline. There were small bone metastases to the pelvis and scapula on bone radionuclide scan, but no other metastasis was observed with subsequent investigations. Modified radical mastectomy including axillary lymph node dissection was performed. Histologically, the excised tumor was consistent with extraskeletal osteosarcoma of the breast accompanied by lymph node metastses. In spite of adjuvant chemotherapy, the patient suffered a local recurrence four months later and died of aggressive multiple metastases 7 months after surgery.