Valentinian I (Latin: Flavius Valentinianus Augustus; 321 – 17 November 375), also known as Valentinian the Great, is Roman emperor from 364 to 375.

Upon becoming emperor he makes his brother Valens his co-emperor, giving him rule of the eastern provinces while Valentinian retains the west.

During his reign, Valentinian fights successfully against the Alamanni, Quadi, and Sarmatians.

Most notable is his victory over the Alamanni in 367 at the Battle of Solicinium.

His brilliant general Count Theodosius defeats a revolt in Africa and the Great Conspiracy, a coordinated assault on Britain by Picts, Scots, and Saxons.

Valentinian is also the last emperor to conduct campaigns across both the Rhine and Danube rivers.

Valentinian rebuilds and improves the fortifications along the frontiers – even building fortresses in enemy territory.

Due to the successful nature of his reign and almost immediate decline of the empire after his death, he is often considered the "last great western emperor".

He founds the Valentinian Dynasty, with his sons Gratian and Valentinian II succeeding him in the western half of the empire.