The Assyrian Church of the East, officially the Holy Apostolic Catholic Assyrian Church of the EastClassical Syriac: ܥܕܬܐ ܩܕܝܫܬܐ ܘܫܠܝܚܝܬܐ ܩܬܘܠܝܩܝ ܕܡܕܢܚܐ ܕܐܬܘܪܝܐ‎ ʻIttā Qaddishtā w-Shlikhāitā Qattoliqi d-Madnĕkhā d-Āturāyē), 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 (Syriac: ܥܕܬܐ ܕܡܕܢܚܐ ʿĒ(d)tāʾ d-Maḏn(ə)ḥāʾ), also known as the Nestorian Church, is a Christian church, part of the Syriac tradition of Eastern Christianity. Originally founded in Assuristan (Persian ruled Assyria), it was the church of the Persian Sassanid Empire, it quickly spread widely through Asia. Between the 9th and 14th centuries it was the world's largest Christian church in terms of geographical extent, with dioceses stretching from the Mediterranean to China and India. Several modern churches claim continuity with the historical Church of the East.

The Church of the East was headed by the Patriarch of the East, continuing a line that, according to tradition, stretched back to the time of the apostles. Liturgically, the church adhered to the East Syrian Rite, and theologically, it is associated with the doctrine of Nestorianism, which emphasizes the distinctness of the divine and human natures of Jesus. This doctrine and its chief proponent, Nestorius (386–451), were condemned by the First Council of Ephesus in 431, leading to the Nestorian Schism and a subsequent exodus of Nestorius' supporters to Sassanid Persia. The existing Christians in Persia welcomed these refugees and gradually adopted Nestorian doctrine, leading the Church of Persia to be known alternately as the Nestorian Church.