Leo V (Leo the Armenian), d. 820, Byzantine emperor (813-20), successor of Michael I. A former general, Leo successfully defended (813) Constantinople against the Bulgars and concluded a 30-year truce with them. Reviving iconoclasm, he deposed the patriarch Nicephorus in the synod of 815 and persecuted the orthodox party led by Theodore of Studium. Leo was murdered by the supporters of his successor, Michael II.