Illinois River, the longest river in Illinois and a navigable tributary of the upper Mississippi River. It is 273 miles (439 km) long. The river is formed in Grundy County, about 45 miles (72 km) southwest of Chicago, by the union of the Des Plaines and Kankakee rivers. It flows westward past Morris, Ottawa, La Salle, and Peru, then bends to the southwest at Depue and passes Lacon, Chillicothe, Peoria, Pekin, Havana, and Beardstown, and finally turns southward near Naples and empties into the Mississippi about 20 miles (32 km) north of St. Louis, Missouri. The river's right-bank tributaries include the Fox, Spoon, and La Moine rivers; those from the left are the Vermilion, Mackinaw, and Sangamon.

Navigation is aided by the Marseilles Canal around the rapids at Marseilles, and by several locks and dams. The Illinois River forms part of the Illinois Waterway between Lake Michigan and the Mississippi River, and is in turn part of the Lakes-to-Gulf Waterway.