The 1838 Mormon War, also known as the Missouri Mormon War, was a conflict between Mormons and non-Mormons in Missouri from August to November 1838, the first of the three "Mormon Wars".

Mormons are a religious and cultural group related to Mormonism, the principal branch of the Latter Day Saint movement started by Joseph Smith in upstate New York during the 1820s. After Smith's death in 1844, the movement split into several groups following different leaders; the majority followed Brigham Young, while smaller groups followed Joseph Smith III, Sidney Rigdon, and James Strang. Most of these smaller groups eventually merged into the Community of Christ, and the term Mormon typically refers to members of the Church of Jesus Christ of Latter-day Saints, as today, this branch is far larger than all the others combined. People who identify as Mormons may also be independently religious, secular, and non-practicing or belong to other denominations. Since 2018, the LDS Church has emphasized a desire for its members be referred to as "members of The Church of Jesus Christ of Latter-day Saints", or more simply as "Latter-day Saints".