system of liturgical practices and discipline historically associated with the Church of the East, or Nestorian Church, and also used today by the Catholic patriarchate of Babylon of the Chaldeans (Chaldean Catholic Church), where it is called the East Syrian rite. Found principally in Iraq, Iran, and Syria, it is also the original rite of the Christians of St. Thomas (Malabar Christians (Christians of Saint Thomas)) in India, established by Nestorian missionaries in the 6th century.

The Chaldean rite originally grew out of the Jerusalem–Antioch liturgy. Its Christians were from Mesopotamia and Chaldea, descendants of the ancient Babylonians, later extending throughout Asia and into India. The term Chaldean was first used in 1445 by Pope Eugenius IV to distinguish the Nestorians of Cyprus, newly reconciled to Rome, from Nestorians proper, henceforth called Assyrians. The term came into popular use following the profession of faith to Rome by John Sulaka, appointed patriarch of “Catholic Nestorians” by Pope Julius III in 1551. The successors of Sulaka later assumed the name Simon and bore the title of “Patriarch-Catholicos of Babylon of the Chaldeans.”

In India, the Malabar Church retained the Syriac language of the Chaldean rite and was governed by Chaldean (Babylonian) bishops. In the modern church, however, the vernacular Malayalam is gradually replacing Syriac as the liturgical language of the Malabarese.

The Chaldean rite, in comparison with other Eastern rites, is simpler in form, lacking, for instance, a detailed lectionary of scriptural verses and commemorating fewer saints. The liturgy is sometimes accompanied with cymbals and triangle and is always chanted.