Illinois Route 9 is a cross-state, east–west rural state highway in central Illinois. It runs from Niota at the Fort Madison Toll Bridge over the Mississippi River to Iowa eastward across central Illinois to State Road 26 at the Indiana border. This is a distance of 218.31 miles (351.34 km).

Illinois 9 is a major arterial route in rural central Illinois. It is a parallel highway to Illinois 116 to the north and U.S. Route 136 to its south.

The road starts at the Indiana border near Cheneyville at Indiana 26 and Indiana 352 and runs as the main highway west, intersecting with Interstate 57 at Paxton; Interstate 55 and Interstate 74 at Bloomington; Interstate 155 at Tremont; and crossing the Illinois River on the John T. McNaughton Bridge between Bartonville and Pekin, where it becomes known as Court Street in the city. It is a two lane highway for most of its length.

Illinois 9 was established in 1918 as one of the original 46 SBI routes. The routing of Illinois 9 has had 2 major changes since its establishment.

The original western terminus was in Hamilton, IL at the old Keokuk Rail Bridge completed in 1916 and then preceded East through Carthage and Macomb (county seats of Hancock and McDonough counties), 9 miles (14 km) east of Macomb, southwest of New Philadelphia, the route turned north to Bushnell and then proceeded east along the current Illinois 9 alignment to Canton, Peoria County, and Pekin.

This current route moved north to terminate in Niota at the Fort Madison Toll Bridge (longest double-deck swing-span bridge in the world) after its completion in July 1928. The route parallels the Atchison, Topeka and Santa Fe Railway and Mississippi River to Dallas City then turns slightly southeast to LaHarpe Illinois. From LaHarpe the route parallels the original Toledo, Peoria and Western Railway right-of way to Bushnell.

From 1935 to 1937, Illinois 9 ran a different route from Pekin to Bloomington, that original route is now posted as: