In human anatomy, the thoracic duct is an important part of the lymphatic system — it is the largest lymphatic vessel in the body. It collects most of the lymph in the body (except that from the right arm and the right side of the chest, neck and head, which is collected by the right lymphatic duct ) and drains into the systemic (blood) circulation.

In adults, the thoracic duct is typically 38-45cm in length. It usually starts from the level of the second lumbar vertebra and extends to the neck.

It originates in the abdomen from the confluence of various abdominal lymph vessels. It extends vertically in the chest and curves posteriorly to the left carotid artery and left jugular vein to empty into the junction of the left subclavian vein and left jugular vein, near the shoulders. It traverses the diaphragm at the aortic aperture and ascends the posterior mediastinum between the descending thoracic aorta (to its left) and the azygos vein (to its right).