Valentinian I (Latin: Flavius Valentinianus; 321 – 17 November 375), sometimes called Valentinian the Great, was Roman emperor from 364 to 375. Upon becoming emperor, he made his brother Valens his co-emperor, giving him rule of the eastern provinces. Valentinian retained the west.

During his reign, Valentinian fought successfully against the Alamanni, Quadi, and Sarmatians. Most notable was his victory over the Alamanni in 367 at the Battle of Solicinium. His general Count Theodosius defeated a revolt in Africa and the Great Conspiracy, a coordinated assault on Roman Britain by Picts, Scots, and Saxons. Valentinian was also the last emperor to conduct campaigns across both the Rhine and Danube rivers. Valentinian rebuilt and improved the fortifications along the frontiers, even building fortresses in enemy territory.

He founded the Valentinianic dynasty, with his sons Gratian and Valentinian II succeeding him in the western half of the empire.