Illinois Route 23 is a north–south state highway in northern Illinois. It runs from Illinois Route 116 in Pontiac north to U.S. Route 14 south of Harvard. This is a distance of 126.17 miles (203.05 km).

Illinois 23 is a major arterial route in rural central and north-central Illinois. It is a parallel to the Interstate 39/U.S. Route 51 combination approximately 20 miles (32 km) to the west.

The road starts near downtown Pontiac and runs as the main highway north, intersecting with Interstate 55 and former U.S. Route 66. It is a two lane highway at this point. Illinois 23 then turns west and runs through Cornell, and turns back north to travel through the middle of Streator. At Ottawa, the road grows to six lanes (across two different roads) when concurrent with Illinois Route 71—Illinois 71 joins with it to cross the Illinois River.

North of Ottawa, Illinois 23 has a full diamond interchange with Interstate 80. This is the last full interchange Illinois 23 has south of its terminus at Harvard. Illinois 23 has two additional concurrencies with U.S. Route 34 and U.S. Route 52, though no notable cities are present between Ottawa and DeKalb. Illinois 23 runs under Interstate 88 with no intersection, though access is available via Fairview Drive to two parallel roads with interchanges (Peace Road and Annie Glidden Road). Between DeKalb and Sycamore, Illinois 23 has four lanes. In Sycamore, Illinois 23 briefly overlaps Illinois Route 64. After one final concurrency at Illinois Route 72 near Genoa, Illinois 23 travels over Interstate 90 with no intersection and no nearby access. The closest way is via U.S. Route 20, and if you are destined to Chicago, turn east. Rockford-Madison, turn west. It runs through Marengo and terminates south of Harvard at U.S. Route 14.

Illinois 23 was established in 1918 as one of the original 46 SBI routes. The routing of Illinois 23 has not changed since its establishment.

Although Route 23 crosses over I-90, there is no exit. Future plans include creating an exit that would open up a faster route to Marengo and Genoa.