Fort Mercer was one of two forts constructed in 1777, on the Delaware River during the American Revolutionary War, by the Continental Army. Built by polish engineer Thaddeus Kosciuszko under the command of George Washington, its purpose was to block the approach to Philadelphia, Pennsylvania: Fort Mifflin, on the Pennsylvania side, and Fort Mercer, on the New Jersey side. Fort Mercer was located in what is now the borough of National Park, Gloucester County, New Jersey, United States. A park, monument, and museum exist today on the site of the fort. The fort was named in honor of Brigadier General Hugh Mercer who had died earlier that year at the Battle of Princeton.