In the U.S. state of Illinois, U.S. Route 36 is an east–west highway that runs across the central portion of the state. It runs east from Missouri over the Mark Twain Memorial Bridge over the Mississippi River with Interstate 72. The eastern terminus of U.S. 36 in Illinois is located near the unincorporated area of Raven near the Illinois-Indiana state line. This is a distance of 216.47 miles (348.37 km).

U.S. 36 parallels Interstate 72 for more than half its routing in Illinois, 133 miles (214 km). It then runs through downtown Decatur and exits Decatur on the southeast side, crossing Lake Decatur before turning due east. After turning east, U.S. 36 runs largely in a straight line to Indianapolis, Indiana.

In downtown Decatur, U.S. 36 is also called Eldorado Street.