Mormon fundamentalism is a conservative movement of Mormonism that believes or practices what its adherents consider to be the fundamental aspects of Mormonism. Most often, Mormon fundamentalism represents a break from the brand of Mormonism practiced by The Church of Jesus Christ of Latter-day Saints (LDS Church), and a return to Mormon doctrines and practices which adherents believe the LDS Church has wrongly abandoned, such as plural marriage, the Law of Consecration, the Adam-God theory, blood atonement, the Patriarchal Priesthood, elements of the Mormon Endowment ritual, and often the exclusion of Blacks from the priesthood. Mormon fundamentalists have formed numerous sects, many of which have established small, cohesive, and isolated communities in areas of the Western United States, as well as Canada and Mexico.

Among the doctrines of Mormon fundamentalism, plural marriage is generally considered the most central and significant doctrine separating fundamentalists from the rest of the Latter Day Saint movement. Plural marriage was practiced by the movement's founder, Joseph Smith, Jr., and by some of his successors in some Latter Day Saint denominations, most notably the LDS Church, which

Most Mormon fundamentalists believe that the doctrine of plural marriage is a fundamental element of Mormonism, and that its renunciation by the LDS church was a mistake. Many Mormon fundamentalists have formed polygynous families, and built remote communities in the West, particularly in Utah and Arizona, where they have little contact from the outside world. Other Mormon fundamentalists, though believing in the doctrine of plural marriage, have not actually taken up the practice.

The LDS Church will excommunicate any of its members who advocate or practice plural marriage, or that actively support fundamentalist groups. Although there continues to be a very small minority of LDS Church members who believe in the doctrine without practicing it, the LDS Church prevents any of its members who sympathize with Mormon fundamentalists from attending its temples.

See also

• Apostolic United Brethren
• Fundamentalist Church of Jesus Christ of Latter Day Saints
• Polygamous Mormon fundamentalists