The hemiazygos vein (Latin: vena hemiazygos, also called vena azygos minor inferior) is a vessel found within the thorax that runs along the right side of the vertebral column. Together with the accessory hemiazygos vein, they serve as the left-sided equivalent of the azygos vein.
The hemiazygos vein is formed by its esophageal and mediastinal tributaries, the common trunk of the left ascending lumbar vein and the left subcostal vein, and by the lower three posterior intercostal veins. It ascends just right to the vertebral column and crosses the column behind the thoracic aorta, esophagus, and the thoracic duct at the level of 8th thoracic vertebra (T8). The hemiazygos vein is often connected to the left renal vein.