Illinois Route 161 (abbreviated IL 161) is an east–west highway with its western terminus at St. Clair Avenue in Fairview Heights and its official eastern terminus at Illinois Route 37 near Kell.

To locals, this part of the highway is known as "The Extension", referring to the fact that road "Extends" on after its official end.

Illinois 161 runs along portions of what once was U.S. Route 50 west of Scott Air Force Base.

East of Belleville, it becomes a rural, two-lane surface road.

SBI Route 161 originally ran from Belleville to Carlyle; in 1941 it was rerouted from Carlyle to IL 37 at its current eastern end, replacing IL 182.