Constitution Island is on the east side of the Hudson River in the U.S. state of New York, north of New York City. It is directly opposite the U.S. Military Academy Reservation at West Point and is connected to the east shore by Constitution Marsh.

It is the only part of the U.S. Military Academy Reservation on the east side of the Hudson River. Formerly known as "Martelaer's Rock", Constitution Island is the site of the earliest Revolutionary War fortifications in the Hudson Valley. Taken briefly by the British in 1777, the island was re-occupied by American forces in 1778, and made an integral part of Fortress West Point. The island was bequeathed to the military academy in 1909 and has been administered by the West Point Museum ever since.

The Education Center was completed in 2016, the Warner House has been completely restored and will be ready as a museum again in the spring of 2020.

Between Cold Spring and West Point lay a large rocky island connected to the eastern shore by a reedy marsh. Dutch Navigators called it Martelaer's Island and the part of the river between it and Storm King Mountain, Martelaer's Rack, or Martyr's Reach. The word "martyr" signified "struggling", as vessels coming up the river with a fair wind would frequently find themselves, immediately after passing the point of the island into the reach, contending with the wind ahead.

The strategic importance of the ability to control navigation along the Hudson River was obvious to both the Americans and the British from the outbreak of open hostilities. The Hudson was the major means for transportation of supplies and troops throughout a large portion of the northeast. On May 25, 1775, the Continental Congress resolved to build fortifications in the Hudson highlands for the purpose of protecting and maintaining control of the Hudson River.

General George Washington was appointed by the Continental Congress to work with the New York Provincial Congress to draft plans as to how the Hudson River should be fortified against the British. Local residents James Clinton and Christopher Tappan were sent to scout appropriate locations. The initial site chosen was Martlaer's Island, across from West Point. Located on a strategic curve in the Hudson River, engineer Bernard Romans, was appointed to begin the construction of a large fort on the island which was to be named “Fort Constitution".

Plans for the fort called for four bastions. Construction of the fortifications began in the summer of 1775, and the island was soon known as Constitution Island. By November it had 70 cannons. However, difficulties in construction and management of the original plan of fortifications, together with escalating costs, led to abandonment of that project. A site at Popolopen Creek across from Anthony's Nose was proposed, and in January 1776, the materials and resources from Fort Constitution were redirected to the construction at the new site. The building of Fort Constitution was suspended while the militia concentrated their efforts on building Forts Clinton, Montgomery and Independence south of the island in the hope of containing the British further downstream. The earthworks at Fort Constitution were unfinished when British troops destroyed them in 1777.

In 1778 Polish engineer Thaddeus Kosciusko began designing Fortress West Point as a series of forts and redoubts on both sides of the river. On Constitution Island, a barracks, three redoubts, and a water battery were constructed.