The Erie Triangle is a tract of American land that was the subject of several competing colonial-era claims and which was eventually acquired by the U.S. federal government and sold to Pennsylvania so that the state would have access to a freshwater port on Lake Erie. The Erie Triangle land makes up a large portion of present-day Erie County, Pennsylvania.

Most of northwestern Pennsylvania came under Anglo-American control following a 1784 treaty with the Iroquois Confederacy. The following year, a boundary dispute between New York and Pennsylvania erupted. Following a surveying effort by Andrew Ellicott representing the Pennsylvanians and James Clinton and Simeon DeWitt representing the New Yorkers, the western edge of New York was set at 20 miles (32 kilometers) east of Pennsylvania's Presque Isle, a small peninsula off the coast of Erie, Pennsylvania. However, this left an unclaimed area, which came to be known as the Triangle Lands.

The Triangle Lands fell under neither New York nor Pennsylvania's charter, while both Connecticut and Massachusetts also spoke up with claims derived from their original colonial sea-to-sea grants.

Of these four competing claimants (Pennsylvania, New York, Connecticut, and Massachusetts), only Pennsylvania was landlocked. Following some pressure from the new federal government, all claims were surrendered to that entity, which then in 1792 turned around and sold final rights to the 202,187 acres (81,822 hectares) of land to Pennsylvania for 75¢ per acre or $151,640.25. Minimal compensation was also provided to Native Americans for their original cession.

The Erie Triangle is often described as a "tab" attached to the Keystone State. The water off the coast of the Erie Triangle is known as the Graveyard of Lake Erie due to the large number of shipwrecks that occurred in the area's stormy waters in the 18th and 19th centuries. It was also the site of a preliminary battle between forces that participated in the Battle of Lake Erie during the War of 1812. The battle itself took place in the Lake Erie Islands , at the western end of the lake.