The Adams-Onís Treaty: How the U.S. Acquired Florida from Spain in 1821

On February 22, 1821, the United States formally took possession of Florida from Spain under the terms of the Adams-Onís Treaty (also known as the Transcontinental Treaty), signed two years earlier in 1819. The agreement, negotiated by Secretary of State John Quincy Adams and Spanish Minister Luis de Onís, resolved long-standing border disputes and transferred both East and West Florida to the U.S. in exchange for the U.S. assuming $5 million in claims by American citizens against Spain and relinquishing claims to Texas.

The transfer ended over 250 years of Spanish colonial rule in the peninsula and marked a major step in the United States' expansion to its continental boundaries. Florida was organized as a U.S. territory in 1822 and admitted as the 27th state in 1845.