An accessory middle cerebral artery is an artery that arises from the anterior cerebral artery and courses parallel to the M1 segment of the middle cerebral artery. This variant has a reported prevalence of 2.7%.

An accessory middle cerebral artery should be distinguished from a duplicated middle cerebral artery. 

The former is a smaller middle cerebral artery branch arising from the anterior cerebral artery while the latter is a smaller middle cerebral artery branch arising from the distal carotid artery.

The accessory middle cerebral artery may provide collateral blood supply to the distal middle cerebral artery territory in the presence of middle cerebral artery occlusion.

Because an aneurysm may occur at the origin of an accessory middle cerebral artery, this variant is clinically much more significant than a duplicated middle cerebral artery.