Introduction Primary extraosseous osteogenic sarcoma is one of the rarest forms of malignant tumor of the breast. It can arise as a result of osseous metaplasia of a preexisting neoplasm or from a none-phyllodes sarcoma of a previously normal breast. Due to its rarity, the natural history and optimal treatment methods remain unclear. Case presentation A 60 years old patient presented to the surgical casualty with a large breast abscess. Abscess wall histology revealed an osteosarcoma of the breast. Left total mastectomy with axillary clearance was performed. Histology and subsequent immunohistochemical studies confirmed the diagnosis of osteogenic sarcoma without lymph nodal metastasis. Patient was referred to the oncologist for further management. Conclusion Rare types of breast tumors can be presented as breast abscess. Incision and drainage together with wall biopsy aid to exclude associated sinister pathologies. Diagnosis of primary osteosarcoma of the breast was made using histological and immunohistochemical findings once the possible primary from the bones of sternum and ribs was excluded. Treatment is as for sarcomas affecting other locations and should comprise a multidisciplinary approach. How to cite this article Nawarathna NJ, Chandrasekara D, Kumarasinghe NR, Balasooriya RS, Rathnayake P, Seneviratne RJ. Primary Osteosarcoma of the Breast presenting as a Large Breast Abscess. J Postgrad Med Edu Res 2016;50(2):96-99.