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.

John Taylor was an English-born religious leader who served as the third president of the Church of Jesus Christ of Latter-day Saints from 1880 to 1887. He is the first and so far only president of the LDS Church to have been born outside the United States.