Illinois Route 40 is a north–south route in central portion of the U.S. state of Illinois. It runs from Interstate 74 in East Peoria north to Illinois Route 78 at Mt. Carroll, just south of U.S. Route 52 and Illinois Route 64. This is a distance of 112.05 miles (180.33 km).

Illinois 40 is known as Knoxville Avenue in the Peoria area, generally south of Illinois Route 6. Route 40 crosses over Interstate 74 north of downtown, and then grows complex in the downtown area, as there are three turns in a one-block length. Illinois 40 then emerges onto William Kumpf Boulevard and crosses over the Illinois River using the Bob Michel Bridge. On the other side of the river, Illinois 40 is briefly called Washington Street. It then is routed onto an onramp to Interstate 74, where there is an exit to Camp Street before terminating at I-74.

Illinois 40 was previously designated Illinois Route 88. In 1992, Illinois Route 5, a four-lane freeway which crosses Illinois 40, was designated Interstate 88. To prevent confusion, IDOT redesignated Illinois 88 as Illinois 40. Although the new number is also the number of a Federal Highway that passes through Illinois (U.S. Route 40), a great enough distance exists between them that the possibility of confusing one for the other is negligible.

Prior to 2003, the Illinois 40 exit on I-74 southeast of Peoria was called "Industrial Spur". As part of the Upgrade 74 Project, the exit was renamed by the Illinois Department of Transportation to Riverfront Drive, presumably to give East Peoria's riverfront exit a better image, and also to reflect the fact that the riverfront is now a commercial destination as opposed to another exit for Caterpillar Inc.

Exactly what was renamed remains obscure, since the onramp has an interchange with Camp Street, terminates at Washington Street, and the only other intersection that Illinois 40 has in East Peoria is with River Road, not Riverfront Drive. The only remaining possibility is that Illinois 40 itself between Washington Street and Interstate 74 is Riverfront Drive. As a result, Illinois 40 has the distinction of being the only onramp explicitly named in Illinois. The name Riverfront Drive is particularly ironic since the road is not the closest road to the river.