Jovian (Latin: Flavius Jovianus Augustus; 331 – 17 February 364), was Roman Emperor from 363 to 364. Upon the death of emperor Julian during his campaign against the Sassanid Persians, Jovian was hastily declared emperor by his soldiers. Jovian sought peace with the Persians on humiliating terms, and reestablished Christianity as the favored religion of the Empire.

Jovian was born at Singidunum (today Belgrade, Serbia) in 331 AD, son of (Flavius?) Varronianus, the commander of Constantius II's imperial bodyguards (comes domesticorum). He also joined the guards, and by 363 had risen to the same command that his father had once held. In this capacity, Jovian accompanied the Roman Emperor Julian on the Mesopotamian campaign of the same year against Shapur II, the Sassanid king. After a small but decisive engagement the Roman army was forced to retreat from the numerically superior Persian force. Julian was mortally wounded during the retreat and died on 26 June 363. The next day, after the aged Saturninius Secundus Salutius, praetorian prefect of the Orient, declined the purple, the choice of the army fell upon Jovian. His election caused considerable surprise, and it is suggested by Ammianus Marcellinus that he was wrongly identified with another Jovianus, chief notary (primicerius notariorum), whose name also had been put forward, or that during the acclamations the soldiers mistook the name Jovianus for Julianus, and imagined that the latter had recovered from his illness.

The Roman emperor was the ruler of the Roman State during the imperial period (starting at about 27 BC). The Romans had no single term for the office although at any given time, a given title was associated with the emperor. If a man was "proclaimed emperor" this normally meant he was proclaimed augustus, or (for generals) imperator (from which English emperor ultimately derives). Several other titles and offices were regularly accumulated by emperors, such as caesar, princeps senatus, consul and Pontifex Maximus. The power of emperors was generally based on the accumulation of powers from republican offices and the support of the army.

Roman emperors refused to be considered "kings", instead claiming to be leaders of a republic, however nominal. The first emperor, Augustus, resolutely refused recognition as a monarch. Although Augustus could claim that his power was authentically Republican, his successor, Tiberius, could not convincingly make the same claim. Nonetheless, the Republican institutional framework (senate, consuls, magistracies etc.) was preserved until the very end of the Western Empire.