Illinois Route 251 is a north–south state highway that runs on the former alignment of U.S. Route 51 before Interstate 39 was built in north central Illinois. It runs from U.S. 51 at the border with Wisconsin to I-39 and U.S. 51 south of Kappa. Illinois 251 is 135.32 miles (217.78 km) long.

U.S. Route 51 (US 51) in the U.S. state of Illinois, is a main north–south artery that runs from the Ohio River north to the Wisconsin border, a distance of 415.95 miles (669.41 km).