Unruptured non-traumatic dissecting aneurysm in the M4 segment of the middle cerebral artery (MCA) accompanied by complete occlusion of the ipsilateral internal cerebral artery (ICA) has never been reported. A 41-year-old man presented with an infarction manifesting as left-sided weakness and dysarthria. Magnetic resonance angiography revealed a subacute stage infarction in the right MCA territory and complete occlusion of the right ICA. Angiography demonstrated aneurysmal dilatation of the M4 segment of the right MCA. Surgery was performed to prevent hemorrhage from the aneurysm. The aneurysm was proximally clipped guided by Navigation-CT angiography and flow to the distal MCA was restored by superficial temporal artery-middle cerebral artery (STA-MCA) anastornosis. We report this rare case with literature review.