Route 21 is a 13.73-mile-long (22.10 km) south–north state highway in Massachusetts that runs between U.S. Route 20 (US 20) and Route 141 in Springfield and Route 9 in Belchertown. Along the way it intersects several major highways including Interstate 90 (I-90) in Ludlow and US 202 and Route 181 in Belchertown.