Flavius Leo V, surnamed the Armenian, a Byzantine emperor, reigned from 813 to 820. He was of noble Armenian descent, distinguished himself as a general under Nicephorus I. (802-811), was exiled for treachery, but soon recalled by Michael L, and appointed commander of the troops in Asia. Michael was chiefly unpopular as the husband of the masculine and presuming Procopia, and his downfall and the elevation of Leo had been foretold by an Asiatic prophetess. In 813 Leo and the emperor led an expedition against the Bulgarians, and were defeated in a battle near Adrianople. Michael withdrew to Constantinople, leaving a disaffected army under the command of Leo, who was the secret cause of the defeat, and whose friends now persuaded the soldiers to proclaim him emperor. The rebel army marched toward the capital, and to avoid civil war Michael resigned to the conspirators the keys of the city and the palace, and retired to a convent. The Bulgarians immediately appeared before Constantinople, desolated its suburbs, captured Adrianople, and reduced Thrace to a desert, but suffered a terrible defeat by Leo at Mesembria in 814. In 815 he invaded their territory, obtained a truce for 30 years, and by his fierce onsets left such an impression on these hereditary enemies of the Byzantine empire that they remained quiet during 74 years.

Educated in a camp, he reformed the civil government by introducing into it the strictness of military discipline, and his incessant oversight and formidable punishments improved the administration both in the capital and the provinces. He protected the iconoclasts, and his severity against the advocates of images created numerous enemies. Michael the Stammerer had contributed largely to his elevation, and had been his stanch adherent, but after repeated warnings was found guilty of treason, and was sentenced to death. On the day appointed for his execution a band of priests and chanters was admitted into the palatial chapel to sing matins. A body of conspirators, friends of Michael, mingled with this procession, in the ecclesiastical habit, with swords under their robes, and at a given signal they rushed upon the emperor, who perished at the altar, after bravely defending himself with the great cross.