State Trunk Highway 21 is a state highway in the U.S. state of Wisconsin. It runs east–west across the center of the state between Sparta and Oshkosh. The highway often serves as a direct route for travelers between Appleton and Oshkosh to Tomah and La Crosse. It is a two-lane surface road for nearly all of its length, with the exception of a few urban arterials of four or more lanes. The section of WIS 21 between Fort McCoy and I-94 in Tomah is classified as a "Major STRAHNET Connector", while the route past I-94 is classified as a "Non-Interstate STRAHNET Route".

Sparta is a city in and the county seat of Monroe County, Wisconsin, United States, along the La Crosse River. The population was 10,025 at the 2020 census.