Leo V the Armenian (Greek: Λέων ὁ Ἀρμένιος, Léōn ho Arménios; c. 775 – 25 December 820) was the Byzantine emperor from 813 to 820. He is chiefly remembered for ending the decade-long war with the Bulgars, as well as initiating the second period of Byzantine iconoclasm.

A senior general of Armenian origin, Leo distinguished himself under Nikephoros I and Michael I Rhangabe, eventually becoming the stratēgos of the Anatolic Theme. Taking advantage of Michael's defeat at the Battle of Versinikia, he forced the emperor to abdicate in his favour. He was able to withhold the blockade of Constantinople by Krum of Bulgaria and, after Krum's death, concluded a 30-year peace with his successor Omurtag.

In 815, Leo deposed Patriarch Nikephoros and reinstituted iconoclasm. He was assassinated by supporters of Michael the Amorian, one of his most trusted generals, who succeeded him on the throne in 820.

Leo was born c. 775 in Umayyad Arminiya, the son of the patrician Bardas, who was of Armenian descent. According to Theophanes Continuatus, Leo was also of Assyrian/Syrian descent. In his youth he fled with his family to the Byzantine Empire and enrolled in the army of the Anatolic Theme. In 802, the general Bardanes Tourkos took over the theme and married one of his daughters to Leo. When Bardanes rebelled, Leo deserted to Emperor Nikephoros I who promoted him to the position of stratēgos of the Armeniac Theme. In 811, when Nikephoros was planning his major campaign against the Bulgars (which was to end disastrously), Arab raiders captured and destroyed the city of Euchaita in the Armeniac Theme—a humiliating defeat in which the salaries of the thematic units were also lost. Nikephoros blamed this on Leo and exiled him. Punishment also included deprivation of his military rank, beating and hair cutting. However, a modern scholar suggests this "Leo" mentioned in the contemporary sources as being punished by Nikephoros was not the same as the later Emperor Leo.

Recalled by Michael I Rhangabe in 811, Leo became governor of the Anatolic Theme and conducted himself well in a war against the Arabs in 812, defeating the forces of the Cilician thughur under Thabit ibn Nasr. Leo survived the Battle of Versinikia in 813 by abandoning the battlefield, but nevertheless took advantage of this defeat to force the abdication of Michael I in his favor on 11 July 813. It was at this time that people assembled at the tomb of Constantine V, an emperor who was victorious against the Bulgars, and cried out to it: "Arise and help the state which is perishing!"

In a diplomatic move, Leo wrote a letter to Patriarch Nikephoros in order to reassure him of his orthodoxy (Nikephoros being obviously afraid of a possible iconoclast revival). A further step in preventing future usurpations was the castration of Michael I's sons. One month later, during his entrance to the Palace quarter, he kneeled before the icon of Christ at the Chalke Gate, which was erected by Empress Irene.

Leo inherited a precarious situation. Within a week of his coronation, Khan Krum of Bulgaria blockaded both Adrianople and Constantinople by land. He agreed to negotiate in person with Krum but used the opportunity to attempt to have him assassinated. The stratagem failed, enraging Krum who sacked the suburbs of Constantinople and towns in southern Thrace. However, he abandoned his siege of the capital, and withdrew to capture and depopulate Adrianople. With this moment of respite, Leo divorced his allegedly adulterous wife and married the daughter of the patrikios Arsaber, the well-regarded Armenian noblewoman Theodosia, crowning her first son, the ten-year-old Symbatios, co-emperor, and renaming him Constantine, recalling the militarily successful iconoclast emperors of the eighth century, Leo III the Isaurian and Constantine V. In 814, Krum sacked Arcadiopolis and other Thracian towns, and planned a full-scale siege of Constantinople, but died of a stroke before he could begin, causing the Bulgar threat to finally recede.

The motives for the second iconoclasm appear to be more straightforward than in the first, and are much less contested among scholars. Like Emperor Philippikos' (r. 711–713) re-introduction of monothelitism in 711, Leo's adoption of iconoclasm was a strategy for imperial survival and a means of bolstering imperial authority (although it is noted that pragmatism and genuine conviction are not mutually exclusive, i.e., Leo may have had genuinely iconoclastic convictions). And just as Patriarch Nikephoros I and Theodore the Stoudite opposed imperial intervention in dogmatic matters, so too did Maximos the Confessor oppose the monothelite policies of Constans II (r. 641–688). In turn, Constans and Leo appealed to the legacy of Constantine the Great in convoking the First Council of Nicaea, and believed their actions were only a re-assertion of this legitimate imperial authority in spiritual matters.