Gliosarcoma is relatively rare brain tumor of glial and sarcomatous origin. The occurrence of this highly malignant brain tumor in the vicinity of the clipped aneurysm has not been reported in the literature. The authors report a case of 37-year-old man who developed sudden onset severe headache. Angiogram displayed a saccular aneurysm located on the bifurcation of left internal carotid artery, and a fusiform aneurysm on the M-1 segment of the left middle cerebral artery. The patient underwent successful surgical clipping of the aneurysms. Three years later, a mass lesion in the vicinity of the clipped aneurysm was detected on magnetic resonance imaging; gliosarcoma was pathologically confirmed after complete tumor extirpation. Despite postoperative radiotherapy and temozolomide chemotherapy, reoccurrence and subarachnoid metastasis unavoidably emerged. Brain injury, co-existence of two pathologies, metal ions released from aneurysm clips, and radiation might have led to tumorigenesis.