Illinois Route 172 is an inverted-L-shaped highway in northwestern Illinois, though it is marked as being a north–south highway on signage.

[1] Illinois 172 is also a part of the Ronald Reagan Trail.

It then runs east over a feeder for the Hennepin Canal to Illinois 40.

Illinois 172 is a two-lane surface road for its entire length.

In 1946, Illinois 172 was used on former Illinois Route 226, which in addition to its current route ran west to Prophetstown, forming a T-shaped route.