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 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.

New York State Route 417 (NY 417) is an east–west state highway located in the Southern Tier of New York in the United States. It begins at exit 20 of the Southern Tier Expressway in the city of Salamanca and ends at a junction with NY 415 in Painted Post, west of the city of Corning. At 105.25 miles (169.38 km) in length, NY 417 is the longest of the state highways that were formerly part of NY 17 before the construction of the Southern Tier Expressway. It also diverges the most from the current NY 17, coming within 100 feet (30 m) of the Pennsylvania state line at one intersection.