Unitarian Universalism (UU or UUism) is a theologically liberal, inclusive belief system. The name resulted from the consolidation of the Unitarian and Universalist denominations in 1961. Unitarian Universalists claim a theological orientation that aspires to creativity, freedom, and compassion with respect for diversity and interconnectedness as well as encouraging spiritual growth and justice-making through fellowship, personal experience, social action, and education. The free and responsible search for truth and meaning serves as a touchstone for Unitarian Universalists. The term "Unitarian Universalism" overlaps with Unitarianism and Universalism, though properly speaking the latter two terms refer historically to separate movements.

The Unitarian Universalist Association (UUA), founded in 1961 as a consolidation of the American Unitarian Association and the Universalist Church in America, is headquartered in Boston, Massachusetts, and serves churches in North America. The UUA represents more than 1,000 member congregations that collectively include more than 217,000 members. Unitarian Universalists follow a congregational model of church governance, in which power resides at the local level to call ministers and make other decisions involving worship, theology and day-to-day church management. The denominational headquarters in Boston in turn provides services for congregations that can more effectively be handled through joint efforts. Founded in 1995, the International Council of Unitarians and Universalists (ICUU) serves congregations throughout the world.

Both Unitarianism and Universalism trace their roots to Christian Protestantism. Many UUs appreciate and value aspects of Christian and Jewish spirituality, but the extent to which the elements of any particular faith tradition are incorporated into one's personal spiritual practices is a matter of personal choice in keeping with UU's creedless, non-dogmatic approach to spirituality and faith development. Even before the Unitarian and Universalist movements combined their efforts at the continental level, the theological significance of Unitarianism and Universalism expanded beyond the traditional understanding of these terms.