The Assyrian Church of the East, officially the Holy Apostolic Catholic Assyrian Church of the East, is a Syriac Church historically centered in Mesopotamia.

It is one of the churches that claim continuity with the historical Patriarchate of Seleucia-Ctesiphon – the Church of the East.

Unlike most other churches that trace their origins to antiquity, the modern Assyrian Church of the East is not in communion with any other churches, either Eastern Orthodox, Oriental Orthodox, or Catholic.The church is headed by the Catholicos-Patriarch of the Assyrian Church of the East, Mar Dinkha IV, who currently presides from Chicago, Illinois, United States.

Below the Catholicos-Patriarch are a number of metropolitan bishops, diocesan bishops, priests, and deacons who serve dioceses and parishes throughout the Middle East, India, North America, Oceania, and Europe (including the Caucasus and Russia).

Theologically, the church is associated with the doctrine of Nestorianism, leading to the church also being known as the "Nestorian Church", though church leadership has at times rejected the Nestorian label.

The church employs the Syriac dialect of the Aramaic language in its liturgy, the East Syrian Rite, which includes three anaphoras, attributed to Saints Addai and Mari, Theodore of Mopsuestia and Nestorius.

The Church of the East developed from the early Assyrian Christian communities in the Assuristan province of the Parthian Empire, and at its height had spread from its Mesopotamian heartland to as far as China and India.

A dispute over patriarchal succession led to the Schism of 1552, resulting in there being two rival Patriarchs.

One of the factions that eventually emerges from this split becomes the modern Assyrian Church of the East, while another becomes the church now known as the Chaldean Catholic Church, which enters into communion with the Catholic Church.