Theodosian Code (thēˌədōˈshən) [key], Latin Codex Theodosianus, Roman legal code, issued in 438 by Theodosius II, emperor of the East. It was at once adopted by Valentinian III, emperor of the West. The code was intended to reduce and systematize the complex mass of law that had been issued since the reign of Constantine I. To a large extent it was based upon two private compilations, the Gregorian ( Codex Gregorianus ) and the Hermogenian ( Codex Hermogenianus ). The Theodosian Code was used in shaping the Corpus Juris Civilis.