Nestorius (; in Ancient Greek: Νεστόριος; c. 386 – c. 450) was the Archbishop of Constantinople from 10 April 428 to August 431. A Christian theologian, several of his teachings in the fields of Christology and Mariology were seen as controversial and caused major disputes. He was condemned and deposed from his see by the Council of Ephesus, the third Ecumenical Council, in 431.His teachings included rejection of the title Theotokos (Mother of God), used for Mary, mother of Jesus, which indicated his preference for the concept of a loose prosopic union of two natures (divine and human) of Christ, over the concept of their full hypostatic union. That brought him into conflict with Cyril of Alexandria and other prominent churchmen of the time, who accused him of heresy.Nestorius sought to defend himself at the Council of Ephesus in 431, but instead found himself formally condemned for heresy by a majority of the bishops and was subsequently removed from his see. On his own request, he retired to his former monastery, in or near Antioch. In 435, Theodosius II sent him into exile in Upper Egypt, where he lived on until 450, strenuously defending his views. His last major defender within the Roman Empire, Theodoret of Cyrrhus, finally agreed to anathematize him in 451 during the Council of Chalcedon.

From then on, he had no defenders within the empire, but the Church of the East never accepted his condemnation. That led later to western Christians giving the name Nestorian Church to the Church of the East where his teachings were deemed Orthodox and in line with its own teachings. Nestorius is revered as among three "Greek Teachers" of the Church (in addition to Diodorus of Tarsus and Theodore of Mopsuestia). The Church of the East's Eucharistic Service, which is known to be among the oldest in the world, incorporates prayers attributed to Nestorius himself.

The Second Council of Constantinople of AD 553 confirmed the validity of the condemnation of Nestorius, refuting the letter of Ibas of Edessa which claimed that Nestorius was condemned without due inquiry.The discovery, translation and publication of his Bazaar of Heracleides at the beginning of the 20th century have led to a reassessment of his theology in western scholarship. It is now generally agreed that his ideas were not far from those that eventually emerged as orthodox, but the orthodoxy of his formulation of the doctrine of Christ is still controversial.