Illinois Route 203 (IL 203) is a 9.51-mile-long (15.30 km) north–south state highway in the southwestern part of the U.S. state of Illinois. It travels from just south of I-55/I-70/US 40 around Gateway International Raceway at Collinsville Road (former US 40) in Fairmont City, north to I-270 near Pontoon Beach. IL 203 is a former segment of US 66.