Unitarianism (from Latin unitas "unity, oneness", from unus "one") is a Nontrinitarian branch of Christianity. Unitarian Christians affirm the unitary nature of God as the singular and unique creator of the universe, believe that Jesus Christ was inspired by God in his moral teachings and that he is the savior of humankind, but he is not comparable or equal to God himself.

Unitarianism was established in order to restore "primitive Christianity before [what Unitarians saw as] later corruptions setting in"; Likewise, Unitarian Christians generally reject the doctrine of original sin. The churchmanship of Unitarianism may include liberal denominations or Unitarian Christian denominations that are more conservative, with the latter being known as biblical Unitarians.

The birth of the Unitarian faith is proximate to the Radical Reformation, beginning almost simultaneously among the Protestant Polish Brethren in the Polish–Lithuanian Commonwealth and in the Principality of Transylvania in the mid-16th century; the first Unitarian Christian denomination known to have emerged during that time was the Unitarian Church of Transylvania, founded by the Unitarian preacher and theologian Ferenc Dávid (c. 1520–1579). Among its adherents were a significant number of Italians who took refuge in Bohemia, Moravia, Poland, and Transylvania in order to escape from the religious persecution perpetrated against them by the Roman Catholic and Protestant churches. In the 17th century, significant repression in Poland led many Unitarians to flee or be killed for their faith. From the 16th to 18th centuries, Unitarians in Britain often faced significant political persecution, including John Biddle, Mary Wollstonecraft, and Theophilus Lindsey. In England, the first Unitarian Church was established in 1774 on Essex Street, London, where today's British Unitarian headquarters is still located.

As is typical of dissenters, Unitarianism does not constitute one single Christian denomination; rather, it refers to a collection of both existing and extinct Christian groups (whether historically related to each other or not) that share a common theological concept of the unitary nature of God. Unitarian Christian communities and churches have developed in Central Europe (mostly Romania and Hungary), Ireland, India, Jamaica, Japan, Canada, Nigeria, South Africa, the United Kingdom, and the United States. In British America, different schools of Unitarian theology first spread in the New England Colonies and subsequently in the Mid-Atlantic States. The first official acceptance of the Unitarian faith on the part of a congregation in North America was by King's Chapel in Boston, from where James Freeman began teaching Unitarian doctrine in 1784 and was appointed rector. Later in 1785, he created a revised Unitarian Book of Common Prayer based on Lindsey's work.