Leo IV the Khazar (Greek: Λέων Δ΄ ὁ Χάζαρος) was Byzantine Emperor from 775 to 780 AD.
Leo was the son of Emperor Constantine V by his first wife, Irene of Khazaria (Tzitzak), the daughter of a Khagan of the Khazars (thought to be Bihar). He was crowned co-emperor by his father in 751. Leo was betrothed to Gisela, daughter of Pepin the Short but the contract was broken. Leo then married Irene, an Athenian from a noble family, in December 769. In 775 Constantine V died, leaving Leo as sole emperor.
On 24 April 776 Leo, following the precedent set by his father and grandfather, appointed his son, Constantine VI, co-emperor. This led to an uprising of Leo’s five half-brothers, including Caesar Nikephoros, who had hoped to gain the throne themselves. The uprising was put down quickly, with the conspirators being beaten, tonsured, and exiled to Cherson under guard.
Leo IV was raised as an iconoclast under his father but was married to Irene, an iconodule. Realizing the division in his realm he pursued a path of conciliation towards the iconodules, previously declared heretical under imperial policy. Leo allowed monks, persecuted and deported under his father, to return to their monasteries, and he was anointed by some among the clergy as “Friend to the Mother of God” for allowing monks to retain images of the Theotokos. In addition to the concessionary actions Leo also appointed an iconophile sympathizer, Paul of Cyprus, to the position of patriarch of Constantinople upon the death of the predecessor. At the end of his reign, Leo reversed his stance of toleration.
Leo’s reign coincided with that of the third Abbasid Caliph, Al-Mahdi, who invaded Byzantine lands on successive occasions from 777–780 before ultimately being repulsed by Leo’s armies, led by generals such as Michael Lachanodrakon. Leo himself set out with his army against the Bulgars but died of fever while on campaign.
Leo’s death on 8 September 780 resulted in the accession of his wife, Irene, to the throne. Theophanes the Confessor records that Leo IV died as a result of a fever brought on from the precious stones in a crown taken from the Hagia Sophia; some scholars have indicated that upon his accession to the throne Leo was already sick, while others believe Leo was murdered by persons unknown, though Irene is suspected.
Constantine VI was the only son of Leo IV and succeeded him as emperor, ruling jointly with his mother, Irene.