Missouri Executive Order 44 (known as the Mormon Extermination Order) was a state executive order issued by Missouri Governor Lilburn Boggs on October 27, 1838, in response to the Battle of Crooked River. The clash had been triggered when a state militia unit from Ray County seized several Mormon hostages from Caldwell County, and the subsequent attempt by the Mormons to rescue them.

Based on exaggerated reports of the battle and rumors of Mormon military plans, Boggs claimed that the Mormons had committed "open and avowed defiance of the law" and had "made war upon the people of Missouri". Governor Boggs directed that "the Mormons must be treated as enemies, and must be exterminated or driven from the State if necessary for the public peace—their outrages are beyond all description".

The order was directed to General John Bullock Clark, and it was implemented by the state militia to forcefully displace the Mormons from Missouri. In response to the order, the Mormons surrendered and subsequently sought refuge in Nauvoo, Illinois. In 1976, citing its unconstitutional nature, Missouri Governor Kit Bond formally rescinded it.

The relationship between the Mormons and the state of Missouri had its roots in 1830, when a group of missionaries were sent to western Missouri with the goal of proselytizing among the Native Americans. This group arrived in Jackson County, Missouri, and initially encountered a welcoming response from some residents who were receptive to their message. In summer of 1831, Jackson County was designated as the place of Zion, a sacred site where Mormons believed they would eventually gather and prepare for the Second Coming of Jesus Christ. However, as the number of Mormons in the area grew, tensions emerged between the Mormons and their non-Mormon neighbors. This was partly due to the religious and cultural differences between the two groups, economic competition, political differences, and fears of cultural displacement.

Tensions reached a boiling point in summer of 1833, when two newspaper articles discussing Missouri laws concerning slavery were published by the Mormon newspaper, the Evening and the Morning Star in Independence, Missouri. These articles were interpreted by Missourians as inviting free blacks to settle in the county. Residents of Jackson County, including several public officials, published a manifesto accusing the Mormons of having a "corrupting influence" on their slaves, and calling for their removal: "peaceably if we can, forcibly if we must." On the same day, July 20, 1833, the W. W. Phelps' printing press, which published the newspaper in Independence, was destroyed by a mob.

Mormons were given a county of their own—Caldwell County—in 1836, following their expulsion from Jackson County in 1833. However, the increasing influx of new converts moving to northwestern Missouri led them to begin settling in adjacent counties. Other settlers, who had operated under the assumption that Mormons would remain confined to Caldwell County, became angry due to these new settlements.

On July 4, 1838, First Presidency member Sidney Rigdon delivered an oration in Far West, the county seat of Caldwell County. Rigdon wanted to make clear that Mormons would meet any attacks on them with force. Far from settling tensions, Rigdon's oration had the opposite effect: it terrified and inflamed the residents of surrounding counties. By the fall of that same year these tensions escalated into open conflict, culminating in the siege of the Mormon settlement in Carroll County, the sacking and burning of Gallatin by the Danites, and the taking of Mormon hostages by Captain Samuel Bogart and his state militia unit, operating in northern Ray County (to the south of Caldwell).

A Mormon armed group from the town of Far West moved south to the militia camp on the Crooked River in order to rescue the hostages, causing rumors of a planned full-scale invasion of Missouri that ran rampant and aroused terror throughout the western part of the state. These rumors only increased as reports of the Battle of Crooked River reached the capital at Jefferson City, with exaggerated accounts of Mormons supposedly slaughtering Bogart's militia company, including those who had surrendered. Further dispatches spoke of an impending attack on Richmond, county seat of Ray County, though in fact no such attack was ever contemplated.