Unitarian Universalism (UUism) is a religion characterized by support for a "free and responsible search for truth and meaning." Unitarian Universalists do not share a creed; rather, they are unified by their shared search for spiritual growth and by the belief that an individual's theology is a result of that search and not obedience to an authoritative requirement. Unitarian Universalists draw on many different theological sources and have a wide range of beliefs and practices.

Both Unitarianism and Universalism have roots in the Christian faith. Historically, Unitarianism referred to the monotheistic belief in the single personhood of God and a rejection of the Christian Trinity; Universalism taught that all souls would achieve salvation and rejected everlasting Hell. Other Christian doctrines were often also disclaimed within Unitarian and Universalist movements. Such beliefs became more prominent during and after the Protestant Reformation, but with antecedant movements and theological debate stretching back to the first centuries of Christianity. Contemporary Unitarian Universalists do not necessarily subscribe to the historic beliefs of Unitarianism and Universalism.

Unitarian Universalism became organized as a religious group in 1961 with establishment of the Unitarian Universalist Association (UUA), a consolidation of the American Unitarian Association and the Universalist Church in America. It is headquartered in Boston, Massachusetts, and serves churches mostly in the United States. The Canadian Unitarian Council became an independent body in 2002.

Other international religious groups have become affiliated with the movement, many of which predate the 1961 consolidation in the United States. Unitarian churches have persisted in Hungary and Transylvania since the sixteenth century and in Britain since the seventeenth. The nineteenth and twentieth centuries witnessed the emergence of related communities around the world, often by single local individuals. Significant populations are now found in northern India, the Philippines, and various African countries. The International Council of Unitarians and Universalists (ICUU), founded in 1995, coordinates national Unitarian and Universalist associations of churches throughout the world. National organizations may include the title "Unitarian Universalist" or "Unitarian" alone.

At the time of the North American merger, the theological significance of these terms had expanded beyond the traditional Christian understanding. Unitarian Universalists today draw from a variety of religious traditions. Individuals may or may not self-identify as Christians or subscribe to Christian beliefs. Unitarian Universalist congregations and fellowships tend to retain some Christian traditions, such as Sunday worship with a sermon and the singing of hymns. The extent to which the elements of any particular faith tradition are incorporated into personal spiritual practice is a matter of personal choice for congregants, in keeping with the creedless, non-dogmatic approach to spirituality and faith development.