Leo I (Latin: Flavius Valerius Leo Augustus) was Eastern Roman Emperor from 457 to 474. A native of Dacia Aureliana near historic Thrace, he was known as Leo the Thracian (Greek: Λέων Α' ὁ Θρᾷξ).
Ruling the Eastern Empire for nearly 20 years, Leo proved to be a capable ruler. He oversaw many ambitious political and military plans, aimed mostly for the aid of the faltering Western Roman Empire and recovering its former territories. He is notable for being the first Eastern Emperor to legislate in Greek rather than Latin.
He is commemorated as a Saint in the Orthodox Church, with his feast day on January 20.