Intimal sarcomas (ISAs) are extremely rare malignant tumors that histologically occur in the tunica intima of large blood vessels of the systemic and pulmonary circulation. Herein, we describe a case of an ISA-based neoplastic aneurysm in the middle cerebral artery (MCA) that resulted in a subarachnoid hemorrhage (SAH). The patient presented to our hospital with severe consciousness disturbance (Glasgow Coma Scale E1V1M2) and anisocoria. On admission, computed tomography (CT) showed a diffuse SAH. At 8 months prior, he presented to a previous hospital with hoarseness. Thoracic CT revealed a threatened rupture of the aorta of the arch. After total arch replacement, he had been diagnosed with ISA from the pathological findings of the resected aorta. Thereafter, he had been treated with adjuvant chemotherapy and radiotherapy without any cerebral vascular imaging studies, before admission at our hospital. Angiogram revealed a multilobar fusiform aneurysm on the right MCA. We performed a superficial temporal artery-MCA anastomosis, trapping, and resection of the affected MCA (including the aneurysm), followed by external decompression. Microscopic hematoxylin-eosin staining showed proliferation of atypical spindle-shaped cells with enlarged nuclei in the lumen of the affected MCA. Immunostaining showed CD31 (±), ERG (+), MDM2 (+), CDK4 (+, slightly), SMA (±), MIB-1 index 13.9%, factor VIII (±), and desmin (-). These pathological findings indicated metastasis of the ISA, which formed the neoplastic aneurysm. An ISA can cause a neoplastic cerebral aneurysm. Therefore, once a patient is diagnosed with an ISA, it is necessary to check periodically the cerebral arteries.