The 21st Senate District of Wisconsin is one of 33 districts in the Wisconsin Senate. Located in southeastern Wisconsin, the district comprises northeast Racine County and southwest Milwaukee County. It includes the city of Franklin, the northern half of the city of Racine, the western half of the city of Greenfield, and part of southwest Milwaukee, as well as the villages of Greendale, Hales Corners, Caledonia, Wind Point, and North Bay.

The Wisconsin Senate is the upper house of the Wisconsin State Legislature. Together with the larger Wisconsin State Assembly they constitute the legislative branch of the state of Wisconsin. The powers of the Wisconsin Senate are modeled after those of the U.S. Senate.