Illinois Route 126 is an east–west state road in northern Illinois. It runs from Illinois Route 47 in Yorkville to Interstate 55 between Plainfield and Bolingbrook. This is a distance of 17.25 miles (27.76 km).

Illinois 126 begins at Illinois 47 in Yorkville, and runs as a rural two-lane road until it intersects historic Lincoln Highway and enters Plainfield as West Lockport Road. Immediately after crossing the DuPage River, it turns northeast onto Main Street, bypassing Downtown Plainfield, and intersects concurrent Illinois Route 59 and US Route 30 at North Division Street. It continues northeastward out of Plainfield on Main Street until it ends at a junction with Interstate 55. There is no access to Illinois 126 from northbound Interstate 55. Likewise, there is no southbound Interstate 55 access from Illinois 126.

For most of its length, Illinois 126 is an undivided two-lane surface road, until a high-speed ramp is used to access Northbound Interstate 55 at its east end.

Congestion in Plainfield is increasing substantially on this highway, Illinois 59, and U.S. 30 because of near-explosive growth in Plainfield, a southwest suburb of Chicago.

The history of Illinois 126, in both of its incarnations, is tied closely to the history of U.S. Route 66.

The original State Bond Issue Route 126 was opened in 1930 to carry the then-new U.S. Route 66 from Springfield to Litchfield, bypassing the numerous towns along Illinois Route 4. Initially this road was marked as both IL 126 and US 66. In 1935 the dual designation of IL 126 was removed, and this road was known only as US 66.

State Bond Issue Route 66 originally ran from Plainfield to Welco Corners in present-day Bolingbrook, where it intersected SBI 4. However by then Illinois SBI 4 was also designated as US 66, which created confusion with IL 66, so in 1935 Illinois 66 was given the route number 126, which had been removed from US 66 in southern Illinois, and the new IL 126 was extended west from Plainfield to Yorkville.

In 1940, US 66 was rerouted through Plainfield to bypass congestion in Joliet, and IL 126 was truncated to run only from Yorkville to Plainfield, with US 66 taking over Illinois Route 126 (SBI 66) from Plainfield to Welco Corners.

When US 66 was rerouted in 1957 onto its new freeway alignment bypassing Plainfield, which would later become I-55, Route 126 was re-extended east from Plainfield along its pre-1940 route to meet US 66/I 55. This resulted in the section of the road from Plainfield to US 66/I 55 having had four route numbers in its history: 66, 126, 66(again), and 126(again).

Until US 30 was rerouted in 2008, IL 126 was concurrent with US 30 for a short distance approaching Plainfield from the west, between Lincoln Highway and IL 126's turn to the northeast onto Main Street.

In the summer of 2012, a total of six people were killed in four separate accidents along IL 126, the most recent on September 14, 2012.