Justinian I (born at 482 – died at 14 November 565), commonly known as Justinian the Great, was Byzantine (east Roman) emperor from 527 to 565. During his reign, Justinian sought to revive the empire's greatness and reconquer the lost western half of the historical Roman Empire.

One of the most important figures of late antiquity and the last Roman emperor to speak Latin as a first language, Justinian's rule constitutes a distinct epoch in the history of the eastern Roman empire. The impact of his administration extended far beyond the boundaries of his time and domain. Justinian's reign is marked by the ambitious but only partly realized renovatio imperii, or "restoration of the Empire".

Because of his restoration activities, Justinian has sometimes been called the "last Roman" in modern historiography. This ambition was expressed by the partial recovery of the territories of the defunct western Roman empire. His general, Belisarius, swiftly conquered the Vandal kingdom in North Africa, extending Roman control to the Atlantic Ocean. Subsequently Belisarius, Narses, and other generals conquered the Ostrogothic kingdom, restoring Dalmatia, Sicily, Italy, and Rome to the empire after more than half a century of rule by the Ostrogoths.

The prefect Liberius reclaimed most of southern Iberia, establishing the province of Spania. These campaigns re-established Roman control over the western Mediterranean, increasing the Empire's annual revenue by over a million solidi. During his reign Justinian also subdued the Tzani, a people on the east coast of the Black Sea that had never been under Roman rule before.

A still more resonant aspect of his legacy was the uniform rewriting of Roman law, the Corpus Juris Civilis, which is still the basis of civil law in many modern states. This work was carried out primarily by his quaestor Tribonian. His reign also marked a blossoming of Byzantine culture, and his building program yielded such masterpieces as the church of Hagia Sophia, which was to be the center of Eastern Orthodox Christianity for many centuries.

A devastating outbreak of bubonic plague (see Plague of Justinian) in the early 540s marked the end of an age of splendour. The Empire entered a period of territorial decline not to be reversed until the 9th century.