We report a case of primary osteosarcoma of the breast which is rare and exhibits poor prognosis. A 52 years-old-woman was admitted with rapidly growing right mammary mass. A huge lobulated dense mass with speckled calcifications, suggesting malignancy, was observed on mammography. She underwent a radical mastectomy. Grossly, the mass measured 16 14 6 cm and showed grayish white hard lobulated tissue with focal hemorrhage and necrosis. Light microscopically, the tumor was confirmed as an osteosarcoma devoid of any epithelial components. In postoperative whole body bone scan, there was no evidence of the other malignancy. To the best of our knowledge, the present case is the first report of primary osteosarcoma of the breast in Korea.