Illinois Route 135 (IL 135) is a 20.31-mile (32.69 km), L-shaped state road in Warren County, Illinois, United States, that runs from IL 164 west of Monmouth to Main Street in Alexis, at the base of the town's water tower.

[1] IL 135 is an undivided two-lane surface road for its entire length.

It runs concurrently with IL 94 for a few miles north of Little York, and with U.S. Route 67 (US 67) for a few miles west of Alexis.

SBI Route 135 originally ran from IL 1 to State Road 154 (SR 154) near Hutsonville via a bridge over the Wabash River.

In 1937 it was applied on what was IL 94B, the current routing.