Illinois Route 138 is a state highway in southwestern Illinois. It runs from Illinois Route 159 north of Bunker Hill to the city of Mount Olive at Illinois Street, a former alignment of U.S. Route 66. This is a distance of 14.95 miles (24.06 km).

Illinois 138 runs east–west, with a bend north through the city of Benld. It is a two-lane surface road for its entire length. South of Benld, it also overlaps Illinois Route 4 for a few miles.

SBI Route 138 ran from Mount Carmel to Grayville; in 1937, this became Illinois Route 1. It was put on its current alignment in 1942, replacing parts of Illinois Route 38.