Illinois 203 could refer to:
- Illinois Route 203, a state highway, formerly part of Illinois Route 3 and U.S. Route 66, near St. Louis
- School districts
- Elwood Community Consolidated School District 203 in Will County
- Naperville Community Unit School District 203 in DuPage County
- New Trier Township High School District 203 in Cook County
- O'Fallon Township High School District 203 in St. Clair County
- Orangeville Community Unit School District 203 in Stephenson County
- Vandalia Community Unit School District 203 in Fayette County
- Westmer Community Unit School District 203 in Mercer County
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