The Valens Aqueduct (Turkish: Bozdoğan Kemeri, meaning "Aqueduct of the grey falcon"; Ancient Greek: Ἀγωγὸς τοῦ ὕδατος, Agōgós tou hýdatos, meaning simply "aqueduct") is a Roman aqueduct which was the major water-providing system of the Eastern Roman capital of Constantinople (modern Istanbul, Turkey). Completed by Roman Emperor Valens in the late 4th century AD, it was restored by several Ottoman Sultans, and is one of the most important landmarks of the city.

The aqueduct stands in Istanbul, in the quarter of Fatih, and spans the valley between the hills occupied today by the Istanbul University and the Fatih Mosque. The surviving section is 921 meters long, about 50 meters less than the original length. The Atatürk Bulvarı boulevard passes under its arches.

The construction of a water supply system for the city (then still called Byzantium) had begun already under the Roman emperor Hadrian. Under Constantine I, when the city was rebuilt and increased in size, the system needed to be greatly expanded to meet the needs of the rapidly growing population.

Valens (^{[pronunciation?]}; Latin: Flavius Julius Valens Augustus; 328 – 9 August 378) was the Eastern Roman Emperor from 364 to 378. He was given the eastern half of the empire by his brother Valentinian I after the latter's accession to the throne. Valens, sometimes known as the Last True Roman, was defeated and killed in the Battle of Adrianople, which marked the beginning of the collapse of the decaying Western Roman Empire.

Valens and his brother Valentinian were both born in Cibalae (in present-day Croatia) into an Illyrian family in 328 and 321 respectively. They had grown up on estates purchased by their father Gratian the Elder in Africa and Britain. While Valentinian had enjoyed a successful military career prior to his appointment as emperor, Valens apparently had not. He had spent much of his youth on the family's estate and only joined the army in the 360s, participating with his brother in the Persian campaign of Emperor Julian.

In February 364, reigning Emperor Jovian, while hastening to Constantinople to secure his claim to the throne, was asphyxiated during a stop at Dadastana, 100 miles east of Ankara. Among Jovian's agents was Valentinian, a tribunus scutariorum. He was proclaimed Augustus on 26 February, 364. Valentinian felt that he needed help to govern the large and troublesome empire, and, on 28 March of the same year, appointed his brother Valens as co-emperor in the palace of Hebdomon. The two Augusti travelled together through Adrianople and Naissus to Sirmium, where they divided their personnel, and Valentinian went on to the West.