Nontrinitarianism includes all Christian belief systems that reject the Trinity, the doctrine that God is three distinct persons in one being.

The absence of the Trinity is not of necessary importance to all nontrinitarians. Persons and groups espousing this position generally do not refer to themselves affirmatively by the term. The Unitarians have adopted a name that bespeaks of their belief in God as subsisting in a theological or cosmic unity. Modern nontrinitarian groups views differ widely on the nature of God, Jesus, and the Holy Spirit.

Various nontrinitarian views, such as adoptionism and Arianism, existed before the Trinity was formally defined as doctrine in AD 325. Nontrinitarianism was later renewed in the Gnosticism of the Cathars in the 11th through 13th centuries, in the Age of Enlightenment of the 18th century, and in Restorationism during the 19th century.