The Mountain Meadows Massacre (September 7–11, 1857) was a series of attacks during the Utah War that resulted in the mass murder of at least 120 members of the Baker–Fancher emigrant wagon train. The massacre occurred in the southern Utah Territory at Mountain Meadows, and was perpetrated by the Mormon settlers belonging to the Utah Territorial Militia (officially called the Nauvoo Legion) who recruited and were aided by some Southern Paiute Native Americans. The wagon train, made up mostly of families from Arkansas, was bound for California, traveling on the Old Spanish Trail that passed through the Territory.

After arriving in Salt Lake City, the Baker–Fancher party made their way south along the Mormon Road, eventually stopping to rest at Mountain Meadows. As the party was traveling west there were rumors about the party's behavior towards Mormons, and war hysteria towards outsiders was rampant as a result of a military expedition dispatched by President Buchanan and Territorial Governor Brigham Young's declaration of martial law in response. While the emigrants were camped at the meadow, local militia leaders, including Isaac C. Haight and John D. Lee, made plans to attack the wagon train. The leaders of the militia, wanting to give the impression of tribal hostilities, persuaded Southern Paiutes to join with a larger party of militiamen disguised as Native Americans in an attack. During the militia's first assault on the wagon train, the emigrants fought back, and a five-day siege ensued. Eventually, fear spread among the militia's leaders that some emigrants had caught sight of the white men, likely discerning the actual identity of a majority of the attackers. As a result, militia commander William H. Dame ordered his forces to kill the emigrants. By this time, the emigrants were running low on water and provisions, and allowed some members of the militia—who approached under a white flag—to enter their camp. The militia members assured the emigrants they were protected, and after handing over their weapons, the emigrants were escorted away from their defensive position. After walking a distance from the camp, the militiamen, with the help of auxiliary forces hiding nearby, attacked the emigrants. The perpetrators killed all the adults and older children in the group, in the end sparing only seventeen young children under the age of seven.

Following the massacre, the perpetrators buried some of the remains but ultimately left most of the bodies exposed to wild animals and the climate. Local families took in the surviving children, with many of the victims' possessions and remaining livestock being auctioned off. Investigations, which were interrupted by the American Civil War, resulted in nine indictments in 1874. Of the men who were indicted, only John D. Lee was tried in a court of law. After two trials in the Utah Territory, Lee was convicted by a jury, sentenced to death, and executed by Utah firing squad on March 23, 1877.

Historians attribute the massacre to a combination of factors, including war hysteria about a possible invasion of Mormon territory and Mormon teachings against outsiders, which were part of the Mormon Reformation period. Scholars debate whether senior Mormon leadership, including Brigham Young, directly instigated the massacre or if responsibility for it lay only with the local leaders in southern Utah.