Mormon fundamentalism is a belief in the validity of selected fundamental aspects of Mormonism as taught and practiced in the nineteenth century, particularly during the administrations of Joseph Smith, Brigham Young, and John Taylor, the first three presidents of the Church of Jesus Christ of Latter-day Saints. Mormon fundamentalists seek to uphold tenets and practices no longer held by mainstream Mormons. The principle most often associated with Mormon fundamentalism is plural marriage, a form of polygyny first taught in the Latter Day Saint movement by the movement's founder, Smith. A second and closely associated principle is that of the United Order, a form of egalitarian communalism. Mormon fundamentalists believe that these and other principles were wrongly abandoned or changed by the LDS Church in its efforts to become reconciled with mainstream American society. Today, the LDS Church excommunicates any of its members who practice plural marriage or who otherwise closely associate themselves with Mormon fundamentalist practices.

Mormonism is the theology and religious tradition of the Latter Day Saint movement of Restorationist Christianity started by Joseph Smith in Western New York in the 1820s and 1830s. As a label, Mormonism has been applied to various aspects of the Latter Day Saint movement, although there has been a recent push from the Church of Jesus Christ of Latter-day Saints to distance themselves from this label. A historian, Sydney E. Ahlstrom, wrote in 1982 that, depending on the context, the term Mormonism could refer to “a sect, a mystery cult, a new religion, a church, a people, a nation, or an American subculture; indeed, at different times and places it is all of these."