{{Infobox U.S. state symbols |Name = Ilinois |Amphibian =Eastern Tiger Salamander |Bird =Cardinal |Butterfly =Monarch Butterfly |Crustacean = |Fish = Bluegill |Flower = Violet |Grass = Big Bluestem |Insect = |Mammal = |Reptile = Painted Turtle |Tree = White oak |Beverage = |Colors = |Dance = |Dinosaur = |Food = |Fossil = |Gemstone = |Instrument = |Mineral = Fluorite |Poem = |StateRock = |Shell = |Ships = |Slogan = Land of Lincoln |Soil = |Song = Illinois |Sport = |Tartan = |Toy = |Route Marker = Illinois 19.svg|300px |Quarter =2003 IL Proof.png|100px|{ } quarter] |QuarterReleaseDate = 2003 }}

The State of Illinois ( , roughly ill-i-NOY) is a state of the United States of America, the 21st to be admitted to the Union. Illinois is the most populous and demographically diverse Midwestern state and the fifth most populous in the nation. With Chicago in the northeast, small industrial cities and great agricultural productivity in central and western Illinois, and natural resources like coal, timber, and petroleum in the south, Illinois has a broad economic base. Illinois is an important transportation hub; the Port of Chicago connects the Great Lakes to the Mississippi River via the Illinois River. Illinois is often viewed as a microcosm of the United States; an Associated Press analysis of 21 demographic factors found Illinois the "most average state," while Peoria has long been a proverbial social and cultural bellwether.

Nearly 66% of the population resides in the northeastern corner of the state comprising the Chicago metropolitan area. 1 in 4 residents of the state live within the city of Chicago itself.

With a population near 40,000 between 1300 and 1400 AD, the Mississippian city of Cahokia, in what is now southern Illinois, was the largest city within the future United States until it was surpassed by New York City between 1790 and 1800. About 2,000 Native American hunters and a small number of French villagers inhabited the Illinois area at the time of the American Revolution. American settlers began arriving from Kentucky in the 1810s; they achieved statehood in 1818. The future metropolis of Chicago was founded in the 1830s on the banks of the Chicago River, one of the only natural harbors on southern Lake Michigan. Railroads and John Deere's invention of the self-scouring steel plow made central Illinois' rich prairie into some of the world's most productive and valuable farmlands, attracting immigrant farmers from Germany and Sweden. Northern Illinois provided major support for Illinoisans Abraham Lincoln and Ulysses S. Grant during the American Civil War. By 1900, the growth of industry in northern cities and coal mining in central and southern areas attracted immigrants from Eastern and Southern Europe, and made the state a major arsenal in both world wars. African-Americans migrating to Chicago from the rural South formed a large and important community, which created the city's famous jazz and blues cultures.