Gratian (^{[pronunciation?]}; Latin: Flavius Gratianus Augustus; 18 April/23 May 359 – 25 August 383) was Roman Emperor from 375 to 383. The eldest son of Valentinian I, during his youth Gratian accompanied his father on several campaigns along the Rhine and Danube frontiers. Upon the death of Valentinian in 375, Gratian's brother Valentinian II was declared emperor by his father's soldiers. In 378, Gratian's generals won a decisive victory over the Lentienses, a branch of the Alamanni, at the Battle of Argentovaria. Gratian subsequently led a campaign across the Rhine, the last emperor to do so, and attacked the Lentienses, forcing the tribe to surrender. That same year, his uncle Valens was killed in the Battle of Adrianople against the Goths – making Gratian essentially ruler of the entire Roman Empire. He favoured Christianity over traditional Roman religion, refusing the divine attributes of the Emperors and removing the Altar of Victory from the Roman Senate.

Gratian was the son of Emperor Valentinian I by Marina Severa, and was born at Sirmium (now Sremska Mitrovica, Serbia) in Pannonia. He was named after his grandfather Gratian the Elder. Gratian was first married to Flavia Maxima Constantia, daughter of Constantius II. His second wife was Laeta. Both marriages remained childless. His stepmother was Empress Justina and his paternal half siblings were Emperor Valentinian II, Galla and Justa.