Justinian I, (483–565), Eastern Roman, or Byzantine, emperor, 527–65. He is called Justinian the Great. Under his rule the Byzantine Empire reached its greatest extent. His most durable achievement was the Justinian Code, or Corpus juris civilis (“Body of Civil Law”), a compilation of Roman law made under his sponsorship. It is the basis of legal systems in many countries today.

Justinian was the nephew of Justin I, whom he succeeded as emperor. Justinian's wife and coruler, Theodora, had a strong influence over him. She probably saved his regime in 532, when she persuaded him not to flee during a revolt, which then collapsed in the face of the royal family's steadfastness.

Under Justinian, many parts of the Roman Empire that had fallen under barbarian rule were reconquered. His general Belisarius retook North Africa from the Vandals, 533–34. In along campaign (535–54), Byzantine armies, first under Belisarius and later under Narses, reconquered Italy from the Ostrogoths. Part of southern Spain was also regained in 554.

Justinian was a lavish builder, erecting churches, theaters, fortresses, monuments, and other public works throughout the empire. His most magnificent work is the Hagia Sophia in Istanbul. Justinian's costly wars and building projects brought ruinous taxation on his subjects, and the news of his death was met with great rejoicing throughout the empire.