The Church of the East (Classical Syriac: ܥܕܬܐ ܕܡܕܢܚܐ, romanized: ʿĒḏtā d-Maḏenḥā ) or the East Syriac Church , also called the Church of Seleucia-Ctesiphon , the Persian Church , the Assyrian Church , the Babylonian Church or the Nestorian Church , was an Eastern Christian church of the East Syriac Rite, based in Mesopotamia. It was one of three major branches of Eastern Christianity that arose from the Christological controversies of the 5th and 6th centuries, alongside the Oriental Orthodox Churches and the Chalcedonian Church (whose Eastern branch would later become the Eastern Orthodox Church). During the early modern period, a series of schisms gave rise to rival patriarchates, sometimes two, sometimes three. Since the latter half of the 20th century, three churches in Iraq claim the heritage of the Church of the East. Meanwhile, the East Syriac churches in India claim the heritage of the Church of the East in India.

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. The Church of the East was headed by the Catholicose of the East seated in Seleucia-Ctesiphon, continuing a line that, according to its tradition, stretched back to the Apostolic Age. According to its tradition, the Church of the East was established by Thomas the Apostle in the first century. Its liturgical rite was the East Syrian rite that employs the Divine Liturgy of Saints 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. Supporters of Nestorius took refuge in Sasanian Persia, where the Church refused to condemn Nestorius and became accused of Nestorianism, a heresy attributed to Nestorius. It was therefore called the Nestorian Church by all the other Eastern churches, both Chalcedonian and non- Chalcedonian, and by the Western Church. Politically the Sassanian and Roman empires were at war with each other, which forced the Church of the East to distance itself from the churches within Roman territory. More recently, the "Nestorian" appellation has been called "a lamentable misnomer", and theologically incorrect by scholars. However, the Church of the East started to call itself Nestorian, it anathematized the Council of Ephesus, and in its liturgy Nestorius was mentioned as a saint. In 544, the general Council of the Church of the East approved the Council of Chalcedon at the Synod of Mar Aba I.

Continuing as a dhimmi community under the Sunni Caliphate after the Muslim conquest of Persia (633–654), the Church of the East 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 Saint Thomas Syrian Christians of Kerala), 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. Around the year 1000, there were more than sixty dioceses throughout the Near East, but by the middle of the 13th century there were about twenty, and after Timur Leng the number was further reduced to seven only. In the aftermath of the division of the Mongol Empire, the rising Buddhist and Islamic Mongol leaderships pushed out and nearly eradicated the Church of the East and its followers. Thereafter, Church of the East dioceses remained largely confined to Upper Mesopotamia and to the Saint Thomas Syrian Christians in the Malabar Coast (modern-day Kerala, India).

Divisions occurred within the church itself, but by 1830 two unified patriarchates and distinct churches remained: the Assyrian Church of the East and the Chaldean Catholic Church (an Eastern Catholic Church in communion with the Holy See). The Ancient Church of the East split from the Assyrian Church of the East in 1968. In 2017, the Chaldean Catholic Church had approximately 628,405 members and the Assyrian Church of the East had 323,300 to 380,000, while the Ancient Church of the East had 100,000.