Illinois Route 203 is a north–south state road in southwestern Illinois. It runs from just south of I-55, I-70, and U.S. 40 around Gateway International Raceway at Collinsville Road (former U.S. 40) in Fairmont City, north to I-270 near Pontoon Beach. This is a distance of 9.51 miles (15.30 km). Illinois 203 is a former segment of U.S. Highway 66.

Illinois 203 is a major north–south artery through the eastern metropolitan St. Louis area.

Illinois 203 replaced an older alignment of Illinois Route 3 in 1964. It originally ran to Hartford, but this was later trimmed back to I-270.