The Church of the East (Classical Syriac: ܥܕܬܐ ܕܡܕܢܚܐ ʿĒḏtā d-Maḏenḥā) or the East Syriac Church, also called the Church of Seleucia-Ctesiphon, the Persian Church, the Assyrian Church, the Babylonian Church, the Chaldean Church or the Nestorian Church, is one of three major branches of Eastern Nicene Christianity that arose from the Christological controversies in the 5th century and the 6th century, alongside that of Miaphysitism (which came to be known as the Oriental Orthodox Churches) and Chalcedonian Christianity (from which Catholicism, Eastern Orthodoxy and Protestantism would arise).

Having its origins in Mesopotamia during the time of the Parthian Empire, the Church of the East developed its own unique form of Christian theology and liturgy. During the early modern period, a series of schisms gave rise to rival patriarchates, sometimes two, sometimes three. In the latter half of the 20th century, the traditionalist patriarchate of the church underwent a split into two rival patriarchates, namely the Assyrian Church of the East and the Ancient Church of the East, which continue to follow the traditional theology and liturgy of the mother church. The Chaldean Catholic Church based in Iraq and the Syro-Malabar Church in India are two Eastern Catholic churches which also claim the heritage of the Church of the East.

The Church of the East organized itself initially in the year 410 as the national church of the Sasanian Empire through the Council of Seleucia-Ctesiphon. In 424, it declared itself independent of the state church of the Roman Empire, which it calls the 'Church of the West'. The Church of the East was headed by the Catholicos-Patriarch of the East seated originally in Seleucia-Ctesiphon, continuing a line that, according to its tradition, stretched back to Thomas the Apostle in the first century. Its liturgical rite is the East Syrian rite that employs the Liturgy of Addai and Mari.

The Church of the East, which was part of the Great Church, shared communion with those in the Roman Empire until the Council of Ephesus condemned Nestorius in 431. The Church of the East refused to condemn Nestorius and was therefore inaccurately called the "Nestorian Church" by those of the Roman Imperial church. More recently, the "Nestorian" appellation has been called "a lamentable misnomer", and theologically incorrect by scholars.

The Church of the East's declaration in 424 of the independence of its head, the Patriarch of the East, preceded by seven years the 431 Council of Ephesus, which condemned Nestorius and declared that Mary, mother of Jesus, can be described as Theotokos "Mother of God." Two of the generally accepted ecumenical councils were held earlier: the First Council of Nicaea, in which an Assyrian bishop took part, in 325, and the First Council of Constantinople in 381. The Church of the East accepted the teaching of these two councils but ignored the 431 Council and those that followed, seeing them as concerning only the patriarchates of the Roman Empire (Rome, Constantinople, Alexandria, Antioch, Jerusalem), all of which were for it "Western Christianity."

Theologically, the Church of the East adopted the dyophysite doctrine of Theodore of Mopsuestia that emphasised the "distinctiveness" of the divine and the human natures of Jesus; this doctrine was misleadingly labelled as 'Nestorian' by its theological opponents.

Continuing as a dhimmi community under the Rashidun Caliphate after the Muslim conquest of Persia (633–654), the Church of the East and its largely Assyrian members played a major role in the history of Christianity in Asia. Between the 9th and 14th centuries, it represented the world's largest Christian denomination in terms of geographical extent, and in the Middle Ages was one of the three major Christian powerhouses of Eurasia alongside Latin Catholicism and Greek Orthodoxy. It established dioceses and communities stretching from the Mediterranean Sea and today's Iraq and Iran, to India, the Mongol kingdoms and Turkic tribes in Central Asia, and China during the Tang dynasty (7th–9th centuries). In the 13th and 14th centuries, the church experienced a final period of expansion under the Mongol Empire, where influential Church of the East clergy sat in the Mongol court.

Even before the Church of the East underwent a rapid decline in its field of expansion in Central Asia in the 14th century, it had already lost ground in its home territory. The decline is indicated by the shrinking list of active dioceses from over sixty in the early 11th century to only seven in the 14th century. In the aftermath of the division of the Mongol Empire, the rising Buddhist and Islamic Mongol leaderships pushed out the Church of the East and its followers in Central Asia. The Chinese Ming dynasty overthrew the Mongols (1368) and ejected Christians and other foreign influences from China, and many Mongols in Central Asia converted to Islam. The Muslim Turco-Mongol leader Timur (1336–1405) nearly eradicated the hitherto large Christian communities in the Middle East. Church of the East Christianity remained largely confined to ethnic Assyrian communities in Upper Mesopotamia, Southeast Anatolia, Northwest Persia and Northeast Levant, and the Saint Thomas Syrian Christians of the Malabar Coast in the Indian subcontinent.^{[citation needed]}