Illinois Route 15 is an east–west highway with its western terminus at Illinois Route 3, U.S. Route 40, I-55, I-64, and I-70 and its eastern terminus at Wabash River at the Illinois/Indiana Border where it meets Indiana 64. This is a distance of 149.64 miles (240.82 km).

West of Mt. Vernon, IL 15 was part of US 460. Until the 1960s, IL 15 went on the IL 160 corridor from Addieville to Okawville, then west on the present IL 177 corridor to Belleville. The section from Nashville to St. Libory was once IL 110, but this was back in the 1940s before US 460 was signed.

The current route was determined in 1967. US 460 was truncated in 1974.