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 major north–south artery through the eastern part of the Greater St. Louis metropolitan area.

[4][5] By 1939, US 67 Alternate was signed after US 67 was rerouted away from East St.

It originally traveled to Hartford, but this was later trimmed back to I-270 as of 1989.