Fort Massac (or Fort Massiac) is a colonial-era fort on the Ohio River in Massac County, Illinois, United States.

Legend has it that, as early as 1540, the Spanish explorer Hernando de Soto and his soldiers constructed a primitive fortification here to defend themselves from hostile native attack. Maps from the early 1700s show an "Ancien Fort" ("Old Fort") near this location.

Fort Massiac was built by the French in 1757, during the French and Indian War. The Fort was originally called "Fort de l’Ascension". The name was changed in 1759, to honor of Claud Louis d'Espinchal, Marquis de Massiac, the French Naval Minister. Massiac is a place which is in the Communes of the Cantal department, in France.

Many sources, going back into the early 1800s, say that "Massac" was derived from "Massacre", a reference to a massacre at the site. One problem with this is that there seems to be no record of such a massacre. This story seems to be an old folk etymology.

Following the end of the French and Indian War in 1763, the French abandoned the fort and a band of Chickasaw burned it to the ground. When Captain Thomas Stirling, commander of the 42nd Highland Regiment, arrived to take possession for Britain, all he found was a charred ruin.

The British never rebuilt the fort. In 1778, during the American Revolutionary War, Col. George Rogers Clark led his regiment of “Long Knives” into Illinois near the fort at Massac Creek. Clark marched overland to Fort Kaskaskia, 100 miles to the north, without firing a shot. From there, Clark marched across Illinois to Fort Sackville (Vincennes), capturing the entire Illinois Territory, and then some, for the State of Virginia.

In 1794, during the Northwest Indian War, President George Washington ordered the fort rebuilt, and for the next 20 years it protected U.S. military and commercial interests in the Ohio Valley. During this time, Fort Massac was the largest outpost of the U.S. Military.

In the Fall of 1803, the Lewis and Clark Expedition stopped at Fort Massac on its way west, recruiting two volunteers.

During the summer of 1805, former U.S. Vice President Aaron Burr and Gen. James Wilkinson allegedly met at Fort Massac, where they drew up plans to personally conquer Mexico and the American southwest.

The Fort was ravaged by the New Madrid Earthquake in 1811-12. It was again rebuilt in time to play a minor role in the War of 1812, only to be abandoned again in 1814. Local citizens dismantled the fort for timber, and by 1828 little remained of the original construction. In 1839 the city of Metropolis was platted about a mile west of the fort.

The Fort Massac site was made a State Park in 1908, becoming the first Illinois State Park. A replica of the old Fort has been built. Each Fall, reenactors gather for the Fort Massac Encampment, acting out life in the 1700s. There is a visitor center located just north of the reconstructed fort that houses a museum and facilities. The museum houses artifacts, Indian points, wax figures in period clothing and tells the history of Fort Massac through exhibits. A miniature scale model is also on display inside the museum. The scale model was built and donated by Mike Gaia of Paducah, KY.