Fort Amsterdam (later Fort George, among other names) was a fortification on the southern tip of Manhattan Island at the confluence of the Hudson and East rivers in what is now New York City. The fort and the island were the center of trade and the administrative headquarters for the Dutch rule of the colony of New Netherland and thereafter British rule of the Province of New York. The fort was the nucleus of the settlement on the island which was at first named New Amsterdam and is central to New York's early history.

Before the fort was constructed, it was the scene where the purchase of Manhattan Island occurred. In its subsequent history, the fort was known under various names such as Fort James, Fort Willem Hendrick and its anglicized Fort William Henry, Fort Anne, and Fort George. The fort changed hands eight times in various battles, the first episode involving Dutch Governor Peter Stuyvesant, who surrendered the fort to Richard Nicolls of England in 1664. In the 18th century, the fort was at the center of the Stamp Act riots in New York when it was used to store shipments of stamped paper brought over from England, while serving as a safe haven for a number of stamp tax collectors. During the American Revolution, it was involved in the Battle of Brooklyn, when volleys were exchanged between the fort and British emplacements on Governor's Island. After 165 years of service, involving a number of conflicts, the fort was finally torn down in 1790 after the American Revolution. After the fort's demolition, the Government House was constructed on the site as a possible house for the United States President. The site is now occupied by the Alexander Hamilton U.S. Custom House, which houses a branch of the National Museum of the American Indian; Bowling Green is nearby.

The construction of the fort marked the official founding date of New York City as recognized by its seal. In October 1683, what would become the first session of the New York legislature convened at the fort. Artillery guns outside the fort overlooking the harbor formed a battery that would later be the namesake of nearby Battery Park.

Starting with the founding and establishment of Fort Amsterdam in 1626, the fort, with its strategic location, overlooking the harbor and the mouth of the Hudson River, and as a lucrative trade center, played a significant role in the history that followed, all the way through the American Revolution, frequently changing hands between the Dutch, British and the Americans, with corresponding changes in the fort's name.

Fort Amsterdam on Manhattan Island, and the greater Dutch colony of New Netherland, was chartered by the Dutch West India Company in 1621. Before Fort Amsterdam was constructed, the Dutch settlers erected a large stone building on the future location of the fort that served as a meeting house. It was surrounded by some 30 makeshift dwellings made from timber and bark, as finished building materials were scarce. The structures were surrounded by red cedar palisades which served as a temporary fort. Manhattan Island at this time was covered by woods and brush, with several marshes about the shore line, and at night the sound of bears and other wildlife could be heard.

During the 17th century, no settlement was secure without a fortification for defense and refuge. Historian Maud Wilder Goodwin maintains the fort existed not so much out of concern for the Indians, but over other ambitious and enterprising Europeans. Historian John Andrew Doyle, however, maintains that the fort, with its earthen walls, was intended more for a defense against Indians and was almost useless as a defense against a professional army or navy at that time. In 1625, after much deliberation, it was decided that the best location for the fort would be at the southern tip of Manhattan Island, with the idea that it would be able to defend against any ship attempting to ascend the Hudson River. The fort was designed by Kryn Fredericksen, deputy and chief engineer of the New Netherland colony. Seventeenth-century Dutch forts all followed a similar design. Probably originally intended as a standard star-shaped fort, Fort Amsterdam had four sides with a bastion at each corner to better protect the walls. The original plan was to face the wall with stone, but this was never accomplished. The fort was built of hard-packed earth or rubble because earthworks would absorb the shock of cannon fire without collapsing, as stone walls might. Fredericksen had instructions to employ the help of nearby Indians in the fort's construction. and were never taken into slavery.

Under the direction of the Dutch West India Company, the first appearance of negro slaves in New Amsterdam occurred in 1626, and were used helping in the construction of the fort, clearing land, farming and other such work. Attitudes about slavery varied among the colonists. Some of the slaves were converted to Christianity, and later some of them were given their freedom and grants of land.

The fort became the nucleus of the New Amsterdam settlement and its mission was protecting New Netherland colony operations in the Hudson River against attack from the English and the French. Although its main function was military, it also served as the center of trading activity. It contained a barracks, a church, a house for the West India Company director, and a warehouse for the storage of company goods.