Arkansas Highway 21 (AR 21) is a north–south state highway in north central Arkansas, in the United States. The route runs 99.14 miles (159.55 km) from U.S. Route 64 (US 64) in Clarksville north across US 62 to the Missouri state line. The route is a two-lane highway with the exception of a brief concurrency with US 62, a four-lane highway, in Berryville.