U.S. Route 31 (US 31) is a part of the United States Numbered Highway System that runs from Spanish Fort, Alabama, to the Mackinaw City, Michigan. In the U.S. state of Indiana, it is a State Road that enters the state via the George Rogers Clark Memorial Bridge between Louisville, Kentucky, and Clarksville, Indiana. The 266.02 miles (428.12 km) of US 31 that lie within Indiana serve as a major conduit. Most of the highway is listed on the National Highway System. Various sections are rural two-lane highway and urbanized four-lane divided expressway. The northernmost community along the highway is South Bend near the Michigan state line.
US 31 was first designated as a US Highway in 1926. A section of the highway originally served as part of the Dixie Highway. US 31 replaced the original State Road 1 (SR 1) designation of the highway which dated back to the formation of the Indiana State Road system. SR 1 ran from Clarksville through Indianapolis to South Bend and ended at the Michigan state line. Realignments and construction projects have expanded the highway to four lanes north of Columbus.