The Territory of Florida was an organized incorporated territory of the United States that existed from March 30, 1822, until March 3, 1845, when it was admitted to the Union as the state of Florida. Originally the major portion of the Spanish territory of La Florida, and later the provinces of East Florida and West Florida, it was ceded to the United States as part of the 1819 Adams–Onís Treaty. It was governed by the Florida Territorial Council.

The first European known to have encountered Florida was Juan Ponce de León, who claimed the land as a possession of Spain in 1513. St. Augustine, the oldest continually inhabited European settlement in the continental U.S., was founded on the northeast coast of Florida in 1565. Florida continued to remain a Spanish possession until the end of the Seven Years' War, when Spain ceded it to the Kingdom of Great Britain in exchange for the release of Havana. In 1783, after the American Revolution, Great Britain ceded Florida back to Spain under the provisions of the Peace of Paris.

The second term of Spanish rule was influenced by the nearby United States. There were border disputes along the boundary with the state of Georgia and issues of American use of the Mississippi River. These disputes were ostensibly solved in 1795 by the Treaty of San Lorenzo, which, among other things, solidified the boundary of Florida and Georgia along the 31st parallel. However, as Thomas Jefferson once predicted, the U.S. could not keep its hands off Florida.

In 1812, United States forces and Georgia "patriots" under General George Mathews unsuccessfully invaded Florida to protect American interests. The "Patriot War" was perceived as ill-advised by many Americans. President James Madison withdrew his support, and the Spanish authorities were promised a speedy exit of the American troops.

The Spanish government offered runaway slaves freedom if they converted to Catholicism and agreed to a term of military service. Under heavy pressure from the U.S., Spain reversed this policy in the late 18th century, to little effect. Slaves continued to flee to Florida, where they were sheltered by the Florida natives, called Seminoles by Americans. They lived in a semi-feudal system; the Seminoles gave the Blacks protection, while the former slaves, who knew how to farm, shared crops with the natives. Although the escaped slaves were still considered inferior by the Seminoles, the two groups lived in relative peace. The slaveholders in Georgia and the rest of the South became furious over this state of affairs as slaves continued to escape to Florida.

In 1818, after years of additional conflicts involving natives, fugitive slaves, and settlers, General Andrew Jackson wrote to President James Monroe, who had been inaugurated in March 1817, informing him that he was invading Florida. Jackson's force departed from Tennessee and marched down to the Florida Panhandle. Spanish officers surrendered coastal fortifications at Fort San Marcos (also known as Fort St. Marks) in Florida Oriental and, about six weeks later, Fort Barrancas and Pensacola in Florida Occidental.

The Adams–Onís Treaty, also known as the Transcontinental Treaty, was signed on February 22, 1819, by John Quincy Adams and Luis de Onís y González-Vara, but did not take effect until after it was ratified by Spain on October 24, 1820, and by the United States on February 19, 1821. The U.S. received Florida under Article 2 and inherited Spanish claims to the Oregon Territory under Article 3, while ceding all its claims on Texas to Spain under Article 3 (with the independence of Mexico in 1821, Spanish Texas became Mexican territory), and pledged to indemnify up to $5,000,000 in claims by American citizens against Spain under Article 11. Under Article 15, Spanish goods received exclusive most favored nation tariff privileges in the ports at Pensacola and St. Augustine for twelve years.

In Dorr v. United States (195 U.S. 138, 141–142 (1904)) Justice Marshall is quoted more extensively as follows: