Louisiana Territory was a historic, organized territory of the United States from July 4, 1805 until December 11, 1812. It consisted of the portion of the Louisiana Purchase that was not partitioned off into Orleans Territory, which later became the state of Louisiana. The Louisiana Purchase however, bought more than the state of Louisiana. The Louisiana Territory was everything in the Purchase north of the 33rd parallel (the southern boundary of the present state of Arkansas). The seat of government was St. Louis.

Louisiana Territory in some contexts can refer to the French or Spanish colonial territories of Louisiana; see French colonization of the Americas, Spanish colonization of the Americas. The term is often used informally as synonymous with all of the area included in the Louisiana Purchase.

Both Meriwether Lewis (1807-1809) and William Clark (1813-1820) served as territorial governors of the Louisiana Territory.

The Louisiana Territory had five subdivisions St. Louis District, St. Charles District, Ste. Genevieve District, Cape Girardeau District and New Madrid District. In 1806, the territorial legislature created the District of Arkansas from lands ceded by the Osage Nation. The remainder was known as the Upper Louisiana Territory.

On October 1, 1812, Governor Clark organized the five administrative districts of Upper Louisiana Territory into counties, which later became the first five counties of Missouri Territory. In 1818, Franklin and Jefferson counties were formed out of the original St. Louis County, leaving St. Louis County with the land that today comprises St. Louis County and St. Louis.

The Louisiana Territory was renamed Missouri Territory in 1812 to avoid confusion with the new state of Louisiana.

See also: Historic regions of the United States