Illinois Route 135 is an L-shaped state road in western Illinois. It runs from Illinois Route 164 west of Monmouth to Main Street in Alexis, at the base of the town's water tower. This is a distance of 20.31 miles (32.69 km).

Illinois 135 is an undivided two-lane surface road for its entire length. It overlaps Illinois Route 94 for a few miles north of Little York, and with U.S. Route 67 for a few miles west of Alexis. Illinois 135 is signed in all four directions, and changes directions at the turnoff from Illinois 94.

SBI Route 135 originally ran from Illinois Route 1 to Indiana State Road 154 near Hutsonville via a bridge over the Wabash River. This was removed in 1935; the bridge is now unnumbered. In 1937 it was applied on what was Illinois Route 94B, the current routing.