The Codex Theodosianus was a compilation of the laws of the Roman Empire under the Christian emperors since 312. A commission was established by Theodosius II in 429 and the compilation was published in the eastern half of the Roman Empire in 438. One year later, it was also introduced in the West by the emperor Valentinian III.

On March 26, 429, Emperor Theodosius II announced to the senate of Constantinople his intentions to form a committee to codify all of the laws (leges, singular lex) from the reign of Constantine up to Theodosius II and Valentinian III. Twenty-two scholars, working in two teams, worked for nine years starting in 429 to assemble what was to become the Theodosian Code. Their product was a collection of 16 books containing more than 2,500 constitutions issued between 313 and 437. John F. Matthews illustrates the importance of Theodosius' Code when he said, "the Theodosian Code was the first occasion since the Twelve Tables on which a Roman government had attempted by public authority to collect and publish its leges." The code covers political, socioeconomic, cultural and religious subjects of the fourth and fifth century in the Roman Empire.