Fort Niagara is a fortification originally built to protect the interests of New France in North America. It is located near Youngstown, New York, on the eastern bank of the Niagara River at its mouth, on Lake Ontario.

René-Robert Cavelier, Sieur de La Salle built the first structure, called Fort Conti, in 1678. In 1687, the Governor of New France, the Marquis de Denonville, constructed a new fort at the former site of Fort Conti. He named it Fort Denonville and posted a hundred men under the command of Capt. Pierre de Troyes, Chevalier de Troyes. The winter weather and disease was severe, and all but twelve perished by the time a relief force returned from Montreal. It was decided in September of 1688 to abandon the post and the stockade was pulled down. In 1726, a two story "Maison a Machicoulis" or "Machicolated House" was constructed on the same site by French engineer Gaspard-Joseph Chaussegros de Lery. It was called the "House of Peace" or trading post to appease the Haudenosaunee, or Iroquois. The name used today, "The French Castle" was not used until the 19th Century. The fort was expanded to its present size in 1755 due to increased tensions between French and British colonial interests.

The fort played a significant part in the French and Indian War, and fell to the British in a nineteen day siege in July 1759, called the Battle of Fort Niagara. The French relief force sent for the besieged garrison was ambushed at the Battle of La Belle-Famille, and the commander of the post, Pierre Pouchot, surrendered the fort to the British commander, Sir William Johnson, who initially led the New York Militia. The Irish-born Johnson was not the original commander of the expedition, but became its leader when General Prideaux literally lost his head, stepping in front of a mortar being test-fired during the siege. The fort remained in British hands for the next thirty-seven years.

Though Fort Niagara was ceded to the United States after the Treaty of Paris ended the American War of Independence in 1783, the region remained effectively under British control for thirteen years. Only after signing of the Jay Treaty did American forces occupy the fort in 1796. In the interim, United Empire Loyalists fleeing persecution in the new USA were given land grants, typically 200 acres (81 ha) per, in Upper Canada and some were sustained in the early years partly by aid from the military stores of the fort. The British captured Fort Niagara during the War of 1812, on the night of December 19, 1813. British forces relinquished it to the United States with the Treaty of Ghent. It has remained in US custody ever since.