 New York State Route 21 | ||||
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 Map of western New York with NY 21 highlighted in red | ||||
| Route information | ||||
| Maintained by NYSDOT and the village of Palmyra | ||||
| Length | 99.99 mi[1] (160.92 km) | |||
| Existed | 1930[2]–present | |||
| Major junctions | ||||
| South end |  NY 417 in Andover | |||
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| North end |  NY 104 in Williamson | |||
| Location | ||||
| Country | United States | |||
| State | New York | |||
| Counties | Allegany, Steuben, Ontario, Yates, Wayne | |||
| Highway system | ||||
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New York State Route 21 (NY 21) is a state highway extending for about one hundred miles (160.9 km) through the western part of New York in the United States. The southern terminus of the route is at an intersection with NY 417 in the village of Andover, and its northern terminus is at a junction with NY 104 in the town of Williamson. In between, NY 21 serves the cities of Hornell and Canandaigua and intersects several major east–west routes, including the Southern Tier Expressway (I-86/NY 17) near Hornell, the conjoined routes of U.S. Route 20 (US 20) and NY 5 in Canandaigua, the New York State Thruway (I-90) in Manchester, and NY 31 in Palmyra.
NY 21 originally extended from the Pennsylvania state line in the south to Lake Ontario in the north when it was assigned as part of the 1930 renumbering of state highways in New York. South of Hornell, the route followed modern NY 36. NY 21 was rerouted to follow its current alignment south of Hornell in the 1950s and truncated on its northern end to Williamson in 1980. Other changes of local importance, mostly realignments to bypass communities along the route, have also occurred at various points in the route's history. NY 21 originally had an alternate route around Canandaigua Lake; however, that highway—designated NY 21A—was eliminated in the 1940s.