Illinois Route 37, or Illinois 37, is a north–south route in southern Illinois. It is 155.07 miles (249.56 km) in length. Originally connecting Effingham to Cairo, it was a heavily-traveled highway before Interstate 57 was opened. That Interstate, which lies very close to most of Illinois 37 (indeed the entire route north of Pulleys Mills), supplanted it as a through route. Illinois 37 was long a part of the most direct route between Chicago and Memphis. Despite its obvious significance in pre-Interstate times, it was never part of the U.S. Highway system. Illinois 37 has become a route of local significance instead of the busy, long-distance, inter-city highway that it was until the 1970s. As a town-to-town route, it especially serves communities which have no access to Interstate 57.

Between Effingham and a point near Watson, a segment in which it shared a route with U.S. Route 45, Illinois 37 has been decommissioned in favor of US 45. Between Watson and just north of Salem, it angles westward and toward the south before meeting US 50 at Salem, Illinois 15 and Illinois 142 at Mount Vernon, Interstate 64 south of Mount Vernon, Illinois 16 and Illinois 34 at Benton, Illinois 149 at West Frankfort, Illinois 13 at Marion, and US 51 and Illinois 3 at its current southern terminus at Urbandale. Both of Illinois Route 148's termini branch from Route 37, the north terminus at Mount Vernon, and the southern terminus just north of Pulley's Mill. It has no direct connection to Interstate 24, which is far better approached from Interstate 57.

All of Illinois 37 is an undivided surface route.