We present the case of a 65-year-old woman with a short-term history of cognitive decline and neuropsychiatric symptoms. Neuroradiological examinations revealed a large left temporo-occipital cystic and calcified tumor mass measuring 6 cm in diameter, which was suspicious for an oligodendroglioma or a choroid plexus carcinoma. Neuropathological investigations finally revealed a gliosarcoma with extensive mesenchymal differentiation. The tumor demonstrated a biphasic pattern consisting of focal anaplastic glial components with vascular proliferation and necrosis. Adjacent sarcomatous tissue displayed pleomorphic fibroblastic cells surrounding metaplastic cartilage and osseous formation. Accounting for only approximately 2% of glioblastomas, gliosarcomas represent a rare entity of intrinsic CNS neoplasms. Exceedingly rare, the mesenchymal part of the gliosarcoma undergoes metaplastic transformation. Interestingly, in our case, the tumor exhibited features of both cartilaginous and osseous differentiation and multifocally showed a sharp transformation zone between highly malignant gliosarcomatous tumor areas and well-differentiated non-proliferative metaplastic regions.