John of Damascus (Arabic: يوحنا الدمشقي, romanized: Yūḥanna ad-Dimashqī; Greek: Ἰωάννης ὁ Δαμασκηνός, romanized: Ioánnēs ho Damaskēnós, IPA: [ioˈanis o ðamasciˈnos]; Latin: Ioannes Damascenus; born Manṣūr ibn Sarjūn, منصور إبن سرجون) or John Damascene was a Christian monk, priest, hymnographer, and apologist. Born and raised in Damascus c. 675 or 676; the precise date and place of his death is not known, though tradition places it at his monastery, Mar Saba, near Jerusalem on 4 December 749.

A polymath whose fields of interest and contribution included law, theology, philosophy, and music, he was given the by-name of Chrysorroas (Χρυσορρόας, literally "streaming with gold", i.e. "the golden speaker"). He wrote works expounding the Christian faith, and composed hymns which are still used both liturgically in Eastern Christian practice throughout the world as well as in western Lutheranism at Easter.

He is one of the Fathers of the Eastern Orthodox Church and is best known for his strong defence of icons. The Catholic Church regards him as a Doctor of the Church, often referred to as the Doctor of the Assumption due to his writings on the Assumption of Mary. He was also a prominent exponent of perichoresis, and employed the concept as a technical term to describe both the interpenetration of the divine and human natures of Christ and the relationship between the hypostases of the Trinity. John is at the end of the Patristic period of dogmatic development, and his contribution is less one of theological innovation than one of a summary of the developments of the centuries before him. In Catholic theology, he is therefore known as the "last of the Greek Fathers".

The main source of information for the life of John of Damascus is a work attributed to one John of Jerusalem, identified therein as the Patriarch of Jerusalem. This is an excerpted translation into Greek of an earlier Arabic text. The Arabic original contains a prologue not found in most other translations, and was written by an Arab monk, Michael, who explained that he decided to write his biography in 1084 because none was available in his day. However, the main Arabic text seems to have been written by an unknown earlier author sometime between the early 9th and late 10th century. Written from a hagiographical point of view and prone to exaggeration and some legendary details, it is not the best historical source for his life, but is widely reproduced and considered to contain elements of some value. The hagiographic novel Barlaam and Josaphat, is a work of the 10th century attributed to a monk named John. It was only considerably later that the tradition arose that this was John of Damascus, but most scholars no longer accept this attribution. Instead much evidence points to Euthymius of Athos, a Georgian who died in 1028.