Valens Aqueduct is a Roman aqueduct located in Istanbul, Turkey. The aqueduct, which was the major water-providing system of the Byzantine capital of Constantinople, was completed by Byzantine Emperor Valens in the late 4th century AD. It was maintained and used by the Byzantines and later the Ottomans. Valens Aqueduct was inscribed as a UNESCO World Heritage Site in 1986 as part of Historic Areas of Istanbul.