The accessory hemiazygos vein (also known as superior hemiazygos vein, Latin: vena hemiazygos accessoria, vena hemiazygos superior) is a vessel that runs along the left side of the vertebral column.
It is mainly formed by veins from the fourth to eighth intercostal spaces on the left side of the body. Sometimes the left bronchial veins also contribute to creating the accessory hemiazygos vein.
The accessory hemiazygos vein descends on the left side of the vertebral column, crosses vertebra T7 and drains in the azygos vein. However, sometimes it flows in the hemiazygos vein. In that case, their common trunk joins the azygos vein.
Both the hemiazygos vein and the accessory hamiazygos vein together serve as the left-sided equivalent of the azygos vein.