The Mormon War is a name sometimes given to the 1838 conflict which occurs between Latter Day Saints (Mormons) and their neighbors in the northwestern region of the U.S. state of Missouri.

This conflict is also sometimes referred to as the Missouri Mormon War to differentiate it from the Utah Mormon War (also known as the "Utah War") and the lesser known Illinois Mormon War.The specific dates of the war are from August 6, 1838 (the Gallatin election battle) to November 1, 1838 when Joseph Smith surrenders at Far West.

During the conflict 22 people are killed (3 Mormons and 1 non-Mormon at Crooked River and 18 Mormons at Haun’s Mill).The conflict is preceded by the eviction of the Mormons from Jackson County, Missouri in 1833.

In 1831, Smith had proclaimed that the county was the Biblical Garden of Eden and that Mormons should establish the City of Zion there.

However, the "old settlers" object to the growing political power of the Mormons, as well as their belief in abolition and the divine destiny of the Native Americans, whom they call Lamanites.All of the conflicts in the Mormon War occur in a corridor 100 miles (160 km) to the east and northeast of Kansas City, Missouri.As a result of the war, nearly all Mormons in Missouri, estimated at more than ten thousand, are forced to leave the state and most settle in or near what will become the city of Nauvoo, Illinois.