Nontrinitarianism is a form of Christianity that rejects the mainstream Christian theology of the Trinity—the belief that God is three distinct hypostases or persons who are coeternal, coequal, and indivisibly united in one being, or essence (from the Ancient Greek ousia). Certain religious groups that emerged during the Protestant Reformation have historically been known as antitrinitarian.

According to churches that consider the decisions of ecumenical councils final, trinitarianism was definitively declared to be Christian doctrine at the 4th-century ecumenical councils, that of the First Council of Nicaea (325), which declared the full divinity of the Son, and the First Council of Constantinople (381), which declared the divinity of the Holy Spirit.

In terms of number of adherents, nontrinitarian denominations comprise a small minority of modern Christians. After the denominations in the Oneness Pentecostal movement, the largest nontrinitarian Christian denominations are the Church of Jesus Christ of Latter-day Saints, Jehovah's Witnesses, La Luz del Mundo, and Iglesia ni Cristo. There are a number of other smaller groups, including Christadelphians, Church of the Blessed Hope, Christian Scientists, Dawn Bible Students, Living Church of God, Assemblies of Yahweh, Members Church of God International, Unitarian Christians, Unitarian Universalist Christians, The Way International, The Church of God International, the United Church of God, Church of God General Conference, Restored Church of God, Christian Disciples Church, and Church of God of the Faith of Abraham.

Nontrinitarian views differ widely on the nature of God, Jesus, and the Holy Spirit. Various nontrinitarian philosophies, such as adoptionism and monarchianism existed prior to the codification of the Trinity doctrine in AD 325, 381, and 431, at the Councils of Nicaea, Constantinople, and Ephesus. Nontrinitarianism was later renewed by Cathars in the 11th through 13th centuries, in the Unitarian movement during the Protestant Reformation, in the Age of Enlightenment of the 18th century, and in some groups arising during the Second Great Awakening of the 19th century.

The doctrine of the Trinity, as held in mainstream Christianity, is not present in the other major monotheistic Abrahamic religions.