Leontius of Byzantium was a Christian monk and theologian active in the 6th century CE, during the reign of Emperor Justinian I. His life is not well-documented, and precise biographical details are uncertain. He is traditionally thought to have been a monk from Palestine who spent time in Constantinople, possibly within a monastery known for its strict orthodoxy. Some modern scholars debate whether "Leontius of Byzantium" and "Leontius of Jerusalem" were the same person or two different theologians.

He was a major participant in the complex Christological debates of his time, writing to defend the doctrinal position established by the Council of Chalcedon (451 CE) against opposing groups such as the Nestorians and Monophysites. Several treatises survive under his name, including Against the Nestorians and Eutychians, Thirty Chapters against Severus, and a collection of theological questions and answers called the Epilyseis.

Leontius's historical importance lies in his method. According to scholars, he skillfully used concepts from Aristotelian and Neoplatonic philosophy, such as substance, nature, and person, to clarify and defend the doctrine that Christ united divine and human natures in a single person. This philosophical approach helped systematize orthodox theology and provided a technical foundation for later Byzantine thinkers. His works were influential for centuries and represent a crucial link between the earlier church councils and the more developed theological syntheses of the later patristic period.