Leontios or Leontius (, ), (d. 705), was Byzantine emperor from 695 to 698. His actual and official name was Leo (Λέων, Leōn), but he is known by the name used for him in Byzantine chronicles.

Leontios was born in Isauria. A professional soldier from an early age, he rose swiftly through the ranks and was appointed strategos (military governor) of the Anatolic theme during the reign of Emperor Constantine IV.

In 686 Leontios was chosen by Justinian II to lead the Byzantine army against the Arabs in Georgia and Armenia. Ruthless even by the standards of the day, Leontios carried the war further into Azerbaijan and Albania. His successes eventually forced the negotiation of a treaty between Byzantium and the Arabic Caliph Abd al-Malik with substantial Arabic concessions and tributes to the Byzantine Emperor.

Leontios was less successful when war against the Arabs was renewed in 692. Leading a substantial Byzantine army, he was defeated at the Battle of Sebastopolis when a large Slavic contingent deserted and left his remaining forces exposed. Furious at the loss of the army, the Emperor Justinian imprisoned Leontius for two years.

The Emperor freed Leontios in 695 and appointed him stategos of the Helladic theme with specific responsibility for recovering land lost to the Arabs. Instead he organized a revolt against the emperor, led largely by his former prison comrades. With the help of the Blue charioteers faction, the Patriarch Kallinikos, and his own military prowess, Leontios soon deposed Justinian and seized the throne himself. Justinian's nose and tongue were slit and he was exiled to Cherson in the Crimea.

During his unpopular reign, Leontios (formally "Leo") refrained from most military operations, attempting to consolidate the empire. This inactivity and defensive posture led to Abd al-Malik dispatching an expedition to take Carthage which fell in 697. Leontios had sent a fleet to retake the city. Rather than return to report their failure they rebelled, overthrowing their admiral and naming a German sailor named Apsimaros as their leader. Apsimaros hastily changed his name to Tiberios and the fleet returned to Constantinople where with the support of the Green charioteers faction, they overthrew Leontios in 698.

In what had by now become a tradition for deposed emperors, Leontios had his nose and tongue slit and was imprisoned in the monastery of Psamathion in Constantinople. When the previous Emperor Justinian returned to the throne in 705, both Tiberios and Leontios were paraded through the streets while the citizenry pelted them with ordure. They were then led to the Hippodrome where they were sentenced to death and executed.