Louisiana Purchase, the purchase of the French province of Louisiana by the United States in 1803. The province stretched from the Mississippi River westward to the Rocky Mountains and from the Gulf of Mexico northward to Canada, covering an area roughly equal to that of the United States prior to the purchase. Except for the Mississippi River on the east and Canada on the north, the boundaries were indefinite. The United States also claimed West Florida between the Mississippi and Perdido rivers as part of the purchase, but Spain denied the claim.
As a result of the purchase, the port of New Orleans and the entire Mississippi River system were secured for American shippers, and the country was free to expand toward the Pacific Ocean. The price was $15,000,000 for an area of 828,000 square miles (2,145,000 km 2 )—less than three cents an acre.