Introduction: Mammary sarcomas are relatively uncommon and they represent less than one percent of all primary breast malignancies. Osteosarcoma of the breast, unassociated with other tumors, is distinctly rare, with published references generally limited to case reports and occasional cases in several series encompassing a heterogeneous group of mammary sarcomas and extraosseous osteosarcomas at various sites. The authors present a patient with pure osteosarcoma of the breast, osteoblastic type, with biologically aggressive pattern. Case Report: A 79-year-old lady became aware of a rapidly enlarging lump in the lateral part of the right breast. Clinical examination revealed a firm to hard, mobile, irregular, and painful breast lump measuring about six by four cm. On examination there was no axillary or supraclavicular lymphadenopathy. After initial diagnosis, excisional biopsy without dissection of the axillary lymph nodes was performed. Therefore, the histological and immunohistochemical findings established the diagnosis of pure primary osteosarcoma of the breast. Conclusion: Pure osteosarcoma of the breast is extremely rare and needs to be distinguished from a variety of benign and malignant breast lesions producing metaplastic bone. Less than a hundred cases of pure osteosarcoma of the breast were reported, but diagnostic confirmation with immunohistochemistry has been performed in relatively few of these cases.