State Trunk Highway 31 (often called Highway 31, STH-31 or WIS 31) is a 22.86-mile (36.79 km) state highway in Kenosha and Racine counties in the US state of Wisconsin that runs north–south as a suburban route passing Racine and Kenosha.

WIS 31 begins at the Illinois state line where it connects to Illinois Route 131 (IL 131), following Green Bay Road to its junction through Pleasant Prairie.

The highway briefly passes through the northwestern part of Kenosha.

WIS 31 passes along the west side of University of Wisconsin–Parkside one mile (1.6 km) south of the Racine County line.

Green Bay Road, named after the historic route connecting Chicago with Milwaukee and Green Bay, has been upgraded to four-lane divided highway for the entire length.