The Saw Mill River is a 23.5-mile (37.8 km) tributary of the Hudson River in Westchester County, New York, United States. It flows from an unnamed pond north of Chappaqua to Getty Square in Yonkers, where it empties into the Hudson as that river's southernmost tributary. It is the only major stream in southern Westchester County to drain into the Hudson instead of Long Island Sound. It drains an area of 26.5 square miles (69 km²), most of it heavily developed suburbia.