Presented herein is a case of primary pure osteosarcoma of the breast. A 59-year-old woman noticed a left breast tumor. Mammography showed a cluster of crushed stone-like calcifications, which gave the tumor a raw cotton-like appearance. Malignancy was suspected on fine-needle aspiration cytology of the tumor. An excisional biopsy was performed. The tumor was 2.0 × 2.4 cm in size. Histopathologically the tumor was composed of diffuse atypical cells with mitosis and a lot of bone. Atypical cells were thought to be neoplastic osteoblasts. Multinucleated osteoclastic cells were interspersed with osteoblasts. Spindle cells were found at the verge of the tumor. A few entrapped tubular structures were seen. Immunohistochemistry indicated that neoplastic osteoblastic cells of the tumor were stained positively for vimentin, but negatively for epithelial markers; which suggested that the tumor cell elements originated from epithelial cells. This tumor was diagnosed as primary extraskeletal osteosarcoma. Partial resection was additionally performed around the excisional biopsy without dissection of the axillary lymph nodes. Bone radionuclide scan after operation showed no abnormal uptake. At 5 years after surgery no recurrence was seen.