The Pocantico River is a nine-mile-long (14 km) tributary of the Hudson River in western central Westchester County, New York, United States. It rises from Echo Lake, in the town of New Castle south of the hamlet of Millwood, and flows generally southwest past Briarcliff Manor to its outlet at Sleepy Hollow. Portions of the towns of Mount Pleasant and Ossining are within its 16-square-mile (41 km2) watershed.