Prospect Bluff Historic Sites (until 2016 known as Fort Gadsden Historic Site, and sometimes written as Fort Gadsden Historic Memorial) is located in Franklin County, Florida, on the Apalachicola River, 6 miles (9.7 km) SW of Sumatra, Florida. The site contains the ruins of two forts.

The earlier and larger one was built by the British in 1814, during the War of 1812. They allowed the members of the disbanded Corps of Colonial Marines, made up largely of fugitive slaves, and Creek tribesmen to occupy it after the British evacuated Florida in 1815, deliberately leaving their munitions behind. At that point, since the British had not named it, Americans started referring to it as Negro Fort. It was destroyed in a river attack from U.S. forces in 1816.

Fort Gadsden was built in 1818 within the former walls of the former Negro Fort.

The site has been known by several other names at various times, including Prospect Bluff, British post, Nicholls' Fort, Blount's Fort, Fort Blount, African Fort, and Fort Apalachicola. The local natives called the land Achackwheithle.

Listed in the National Register of Historic Places and named a National Historic Landmark in 1972, the Prospect Bluff Historic Sites was acquired by the Apalachicola National Forest in 1940 and is managed by the U.S. Forest Service. The process of memorializing the site began in 1961, when the Apalachicola National Forest issued the State of Florida a term special use permit for an area of approximately 78 acres (32 ha) including the site to be run as a state park. Administration of the site reverted to the federal government in the 1990s. The site contains interpretive signage, picnic area with pavilion, and rest rooms.

The site of both Negro Fort and the later Fort Gadsden was Prospect Bluff, "a fine bluff overlooking the Apalachicola River," whose modest elevation of 12 feet (3.7 m) and the swamp that almost surrounded it (description below) gave it a natural military strength. The name parallels the Spanish one, Loma de Buena Vista, literally "hill with a good view".

Accessible only by river then, the site was and is still remote. The river was the boundary between East Florida and West Florida during the British Florida period (1763–1783) and the second Spanish Florida period (1783–1821). By modern land route it is 198 miles (319 km) from Pensacola and 271 miles (436 km) from St. Augustine. The area was sparsely populated, and in the twentieth century a large portion became the Apalachicola National Forest. The river was of intense interest to the British, who saw it as an undefended entry into the United States through Georgia. It was of no interest to the Spaniards; it led nowhere they cared about. Spanish forces in Florida were limited and Spain was far less committed to Florida than it was to its other colonies, most of them much more productive. Spain's inability to police its borders or return fugitive slaves was central to Florida's transfer to the United States in 1821.

Control of Prospect Bluff meant control of the river, which had served as a transportation artery for centuries. More recently, it had enabled raiding parties to go upriver into Georgia and the Mississippi Territory via the Chattahoochee and especially the Flint River. The attacks were made on plantations, which had few if any defenses. These parties, besides coming back with material goods, saw to it that the slaves of the raided plantations could get free. This was a great economic blow to the slave owners (slaves were expensive), and an ideological affront as well, leaving them insecure and angry.