WIS 31 begins at the Illinois state line where it connects to ILL 131, following Green Bay Road to its junction through Pleasant Prairie. The highway briefly passes through the northwestern part of Kenosha. WIS 50 and WIS 158 cross WIS 31 in Kenosha. WIS 31 passes along the west side of University of Wisconsin–Parkside one mile (1.6 km) south of the Racine County line. In the Racine area, WIS 31 crosses WIS 11, WIS 20 and WIS 38, turning onto Ole Davidson Road in Caledonia. 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. Ole Davidson Road has been upgraded to a divided highway from the split with North Green Bay Road to Four Mile Road.