| State: | IL |
| Type: | IL |
| Maint: | IDOT |
| Route: | 123 |
| Map Custom: | yes |
| Map Notes: | IL 123 highlighted in red |
| Length Mi: | 33.72 |
| Length Round: | 2 |
| Length Ref: | [1] |
| Direction A: | West |
| Direction B: | East |
| Terminus A: | in Pleasant Plains |
| Junction: | in Williamsville |
| Terminus B: | in Williamsville |
| Counties: | Sangamon, Menard |
| Established: | 1924[2] |
| Previous Route: | 122 |
| Previous Type: | IL |
| Next Route: | 124 |
| Next Type: | IL |
Illinois Route 123 (IL-123) is an east - west state highway in central Illinois, USA. 33.72miles long, it stretches from Historic Route 66 at Williamsville to Illinois Route 125 near Pleasant Plains.
All of IL-123's route is contained within Sangamon and Menard counties. Major towns located on or adjacent to IL-123 include Athens, Petersburg, and Williamsville.
New Salem, the home of Abraham Lincoln in the 1830s, has been reconstructed as Lincoln's New Salem State Historic Site near Petersburg on IL-123.
SBI Route 123 was what Illinois 123 is now, plus a road from Ashland south to Alexander at Interstate 72/U.S. Route 36. In 1999, Illinois 123 was truncated on its southern end to Illinois 125. In October 2003, Illinois 123 was extended east to Williamsville, replacing some of Illinois Route 124 in the process as well as all of Sangamon County Route 11.[2]