Illinois (Wikipedia) Illinois (/ˌɪləˈnɔɪ/ IL-ə-NOY) is a state in the Midwestern United States. Chicago is its largest city, and the state’s capital is Springfield; other major metropolitan areas include Metro East (of Greater St. Louis), Peoria and Rockford. Of the fifty U.S. states, Illinois has the fifth largest gross domestic product (GDP), the sixth largest population, and the 25th largest land area.