The Louisiana Purchase encompassed portions of 15 current U.S. states and 2 Canadian Provinces. The land purchased contained all of present-day Arkansas, Missouri, Iowa, Oklahoma, Kansas, Nebraska, parts of Minnesota that were south of the Mississippi River, most of North Dakota, nearly all of South Dakota, northeastern New Mexico, northern Texas, the portions of Montana, Wyoming, and Colorado east of the Continental Divide, and Louisiana west of the Mississippi River, including the city of New Orleans. (The Oklahoma Panhandle, and southwestern portions of Kansas and Louisiana were still claimed by Spain at the time of the Purchase.)

In addition, the Purchase contained small portions of land that would eventually become part of the Canadian provinces of Alberta and Saskatchewan. The land included in the purchase comprises around 23% of the territory of the United States today.

The purchase was an important moment in the presidency of Thomas Jefferson. At the time, it faced domestic opposition as being possibly unconstitutional. Although he felt that the U.S. Constitution did not contain any provisions for acquiring territory, Jefferson decided to purchase Louisiana because he felt uneasy about France and Spain having the power to block American trade access to the port of New Orleans.