New York State Route 420 (NY 420) is a north–south state highway in St. Lawrence County, New York, in the United States. The southern terminus of the route is at an intersection with U.S. Route 11 (US 11) in the town of Stockholm. Its northern terminus is at a junction with NY 37 in the village of Massena. NY 420 passes through undeveloped regions of St. Lawrence County for most of its routing and briefly overlaps with NY 11C in the hamlet of Winthrop.