| State: | IL |
| Type: | IL |
| Route: | 203 |
| Alternate Name: | Nameoki Road |
| Map Custom: | yes |
| Map Notes: | IL 203 highlighted in red |
| Length Mi: | 9.51 |
| Length Ref: | [1] |
| Direction A: | South |
| Terminus A: | Collinsville Road in Fairmont City |
| Junction: | in Fairmont City |
| Direction B: | North |
| Terminus B: | in Granite City |
| Counties: | St. Clair, Madison |
| Established: | 1964[2] |
| Previous Route: | 192 |
| Previous Type: | IL |
| Next Type: | IL |
| Next Route: | 242 |
Illinois Route 203 (IL 203) is a 9.51adj=midNaNadj=mid 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.
IL 203 is a major north–south artery through the eastern part of the Greater St. Louis metropolitan area.
By 1937, US 67 was signed on part of present-day IL 203.[3] From 1938 to 1955, US 66 City was signed on this particular route as well.[4] By 1939, US 67 Alternate was signed after US 67 was rerouted away from East St. Louis.[5] Up until 1964, parts of IL 3, IL 162, and US 67 Alternate utilized present-day IL 203. In 1964, IL 203 was signed; replacing these routes. It originally traveled to Hartford, but this was later trimmed back to I-270 as of 1989.[2]