The Assyrian Church of the East (ACOE), sometimes called the Church of the East and officially known as the Holy Apostolic Catholic Assyrian Church of the East, is an Eastern Syriac Christian denomination that follows the traditional Christology and ecclesiology of the historical Church of the East. It belongs to the eastern branch of Syriac Christianity, and employs the Divine Liturgy of Saints Addai and Mari belonging to the East Syriac Rite. Its main liturgical language is Classical Syriac, a dialect of Eastern Aramaic. Officially known as the Church of the East until 1976, it was then renamed the Assyrian Church of the East, with its patriarchate remaining hereditary until the death of Shimun XXI Eshai in 1975.

The Assyrian Church of the East is officially headquartered in the city of Erbil, in northern Iraq; its original area encompassed Iraq, southeastern Turkey, northeastern Syria and northwestern Iran, corresponding roughly to ancient Assyria. The current Catholicos-Patriarch of the Assyrian Church of the East, Mar Awa III, was consecrated in September 2021.

The Assyrian Church of the East claims continuity with the historical Church of the East, and is not in communion with the Catholic, Oriental Orthodox or Eastern Orthodox churches. The faction of the original Church of the East that came into full communion with the Holy See between the 16th and 19th centuries is the Chaldean Catholic Church, whose members are mostly ethnic Assyrians. After the Common Christological Declaration in 1994 between the Church of the East and the Catholic Church, and a 2001 theological dialogue between the churches, they drew up guidelines for the faithful to have mutual admission to the Eucharist between the Chaldean Catholic Church and the Assyrian Church of the East.

The Assyrian Church of the East has a traditional episcopal structure, headed by the Catholicos-Patriarch. Its hierarchy is composed of metropolitan bishops and diocesan bishops, while lower clergy consists of priests and deacons, who serve in dioceses (eparchies) and parishes throughout Western Asia, India, North America, Oceania, and Europe (including the Caucasus and Russia).

The Assyrian Church of the East considers itself as the continuation of the Church of the East, a church that originally developed among the Assyrians during the first century AD in Assyria, Upper Mesopotamia and northwestern Persia, east of the Byzantine Empire. It is an apostolic church established by Thomas the Apostle, Addai of Edessa, and Bartholomew the Apostle.

The distinction between the Assyrian Church of the East and the Chaldean Catholic Church resulted from the series of complex processes and events that occurred within the Church of the East during the transitional period that started in the middle of the 16th century, and lasted until the beginning of the 19th century. That turbulent period was marked by several consequent splits and mergers, resulting in the creation of separate branches and rival patriarchal lines. During the entire period, one of the main questions of dispute was the union with the Catholic Church. Ultimately, the pro-Catholic branches were consolidated as the Chaldean Catholic Church, while the traditional branches were consolidated as the Assyrian Church of the East.

During the patriarchal tenure of Shemon VII Ishoyahb (1539–1558), who resided in the ancient Rabban Hormizd Monastery near Alqosh, an internal dissent occurred over several issues, including the question of hereditary succession to the patriarchal throne, and the question of union with the Catholic Church. By that time, Franciscan missionaries had already gained some influence over several local communities, and they took an active role in organizing the opposition to the patriarch at that time. By the end of 1552, a pro-Catholic party had been organized in Mosul under the leadership of the priest Yohannan Sulaqa, who decided to legitimize his position by traveling to Rome and seeking confirmation by Pope Julius III (1550–1555).

Receiving support from the Franciscan missionaries, he arrived in Rome and entered into full communion with the Catholic Church in February 1553. At that point, officials of the Roman Curia were given incorrect information that the elderly Patriarch Shemon VII had actually died. After some deliberation, the Roman pope decided to appoint Yohannan Sulaqa as "Patriarch of Babylon" and named the breakaway church as 'The Church of Assyria and Mosul" in April 1553.