The behavior of phyllodes tumors is unpredictable and can behave as benign fibroadenomas or malignant neoplasms mimicking the course of aggressive sarcomas, characterized by distant metastases and a high short-term mortality. The malignant forms are treated with surgery and adjuvant chemotherapy, but often with poor outcomes. We examine the first reported case of an aggressive osteosarcoma subtype of phyllodes tumor that recurred in the skull after total mastectomy and adjuvant chemotherapy. The skull lesion was treated with excision, and the patient currently remains disease-free.