Route 13 is a highway in Missouri which runs almost the entire north–south length of the state. Its northern terminus is at U.S. Route 69/136 in Bethany. Its southern terminus is at the Arkansas state line in downtown Blue Eye, Missouri–Arkansas where it continues as Highway 21. It is one of the original state highways of Missouri.

Highway 21 is a north–south state highway in north central Arkansas. The route of 99.14 miles (159.55 km) runs from US Route 64 (US 64) in Clarksville north across US 62 to Missouri Route 13 at 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.