State Trunk Highway 51 (often called Highway 51, STH-51 or WIS 51) was a number assigned to two different state highways in the U.S. state of Wisconsin: * Highway 51 from 1917 to 1923, along the current route of U.S. Highway 63 from Turtle Lake to Spooner. * Highway 51 from 1923 to 1936, currently routed as Highway 72 from Ellsworth to Downsville. * For the highway in Wisconsin numbered 51 since 1926, see U.S. Highway 51.