Valentinian I (Latin: Valentinianus; 321 – 17 November 375), sometimes known as Valentinian the Great, was Roman emperor from 364 to 375. He ruled the Western half of the empire, while his brother Valens ruled the East. During his reign, he fought successfully against the Alamanni, Quadi, and Sarmatians, strengthening the border fortifications and conducting campaigns across the Rhine and Danube. Also, his general Theodosius the Elder defeated a revolt in Africa and the Great Conspiracy (a coordinated assault on Roman Britain by Picts, Scoti, and Saxons). Valentinian founded the Valentinian dynasty, with his sons Gratian and Valentinian II succeeding him in the western half of the empire and his daughter Galla marrying emperor Theodosius I.

Valentinian was born in 321 at Cibalae (now Vinkovci, Croatia) in southern Pannonia into a family of Illyro-Roman origin. Valentinian and his younger brother Valens were the sons of Gratianus (nicknamed Funarius), a military officer renowned for his wrestling skills.

Gratianus was promoted to comes Africae in the late 320s or early 330s, and the young Valentinian accompanied his father to Africa. However, Gratianus was soon accused of embezzlement and retired. Valentinian joined the army in the late 330s and later probably acquired the position of protector domesticus. Gratianus was later recalled during the early 340s and was made comes Britanniarum. After holding this post, he retired to the family estate in Cibalae.

In 350, Constans I was assassinated by agents of the usurper Magnentius, a commander who proclaimed himself emperor in Gaul. Constantius II, older brother of Constans and emperor in the East, promptly set forth towards Magnentius with a large army. The following year the two emperors met in Pannonia and fought the Battle of Mursa Major, which ended in a costly victory for Constantius. Two years later Magnentius killed himself after another defeat at the Battle of Mons Seleucus, leaving Constantius sole ruler of the empire. It was around this time that Constantius confiscated Gratianus' property, for supposedly showing hospitality to Magnentius when he was in Pannonia. Despite his father's fall from favour, Valentinian does not seem to have been adversely affected at this time, making it unlikely he ever fought for the usurper. It is known that Valentinian was in the region during the conflict, but what involvement he had in the war, if any, is unknown.

During the conflict between Magnentius and Constantius, the Alamanni and Franks took advantage of the confusion and crossed the Rhine, attacking several important settlements and fortifications. In 355, after deposing his cousin Gallus but still feeling the crises of the empire too much for one emperor to handle, Constantius raised his cousin Julian to the rank of Caesar. With the situation in Gaul rapidly deteriorating, Julian was made at least nominal commander of one of the two main armies in Gaul, Barbatio being commander of the other. Constantius devised a strategy where Julian and Barbatio would operate in a pincer movement against the Alamanni. However, a band of Alamanni slipped past Julian and Barbatio and attacked Lugdunum (Lyon). Julian sent the tribunes Valentinian and Bainobaudes to watch the road the raiders would have to return by. However, their efforts were hindered by Barbatio and his tribune Cella. The Alamanni king Chnodomarius took advantage of the situation and attacked the Romans, inflicting heavy losses. Barbatio complained to Constantius and the debacle was blamed on Valentinian and Bainobaudes, who were cashiered from the army.

With his career in ruins, Valentinian returned to his new family estate in Sirmium (in modern day Serbia). Two years later his first wife Marina Severa had given birth to a son, Gratian. During the (sole) reign of the polytheist Julian, Valentinian's actions and location become uncertain, but he was likely exiled, or perhaps simply sent to command a remote outpost. The sources give contradictory accounts of what happened, with some versions saying that he refused to make pagan sacrifices and voluntarily chose to leave, and others saying he was merely dismissed for his Christianity.

At the news of Julian's death on a campaign against the Sasanian Empire, the army hastily declared Jovian the new emperor. Jovian extricated his soldiers from Persian territory by agreeing to a humiliating peace treaty, then started back to Constantinople. During Jovian's brief reign Valentinian was promoted to tribune of a Scutarii (elite infantry) regiment, which Hughes considered to reflect the new emperor’s trust in him, and dispatched to Ancyra (modern day Ankara).

Jovian died in mysterious circumstances before he reached the capital, and a meeting of civil and military officials was convened at Nicaea to choose a new emperor. Salutius, who had already refused the throne after Julian's death, now did so again, first for himself and then on behalf of his son. Two other names were proposed: Aequitius, a tribune of the first Scutarii, and Januarius, a relative of Jovian's in charge of military supplies in Illyricum. Both were rejected; Aequitius as too rough and boorish, Januarius because he was too far away. As a man well qualified and at hand nearby, the assembly finally agreed upon Valentinian and sent messengers to inform him in Ancyra.