Illinois Route 91 is a rural state road in central Illinois. It runs from the northwest edge of Peoria at U.S. Highway 150 to U.S. Highway 34 south of Kewanee. Illinois 91 is 40.72 miles (65.53 km) long.

Illinois 91 serves the cities of Toulon, Wyoming, Princeville, and Dunlap. It overlaps Illinois Route 90 around Princeville.

Illinois 91 does not run in a straight line between any two major towns. Illinois 91's endpoints are generally northwest-to-southeast, but the route consists entirely of north–south and east–west stretches.

The Rock Island Trail State Park follows Illinois 91 for much of its length. This State Park trail is the former right-of-way of a Peoria branch line of the Chicago, Rock Island and Pacific Railroad.

SBI Route 91 was the current U.S. Route 150 from Knoxville to Peoria. In 1941, the route number was moved to what was then Illinois Route 30, and has remained so to this day.