Illinois ( IL-in-OY) is a state in the Midwestern region of the United States. It borders Lake Michigan to its northeast, the Mississippi River to its west, and the Wabash and Ohio rivers to its south. Of the fifty U.S. states, Illinois has the fifth-largest gross domestic product (GDP), the sixth-largest population, and the 25th-most land area. Its capital city is Springfield in the center of the state, and the state's largest city is Chicago in the northeast.

Present-day Illinois was inhabited by Indigenous cultures for thousands of years. The French were the first Europeans to arrive, settling near the Mississippi and Illinois River in the 17th century Illinois Country, as part of their sprawling colony of New France. A century later, the revolutionary war Illinois campaign prefigured American involvement in the region. Following U.S. independence in 1783, which made the Mississippi River the national boundary, American settlers began arriving from Kentucky via the Ohio River. Illinois was soon part of the United States' oldest territory, the Northwest Territory, and in 1818 it achieved statehood. The Erie Canal brought increased commercial activity in the Great Lakes, and the invention of the self-scouring steel plow by Illinoisan John Deere turned the state's rich prairie into some of the world's most productive and valuable farmland, attracting immigrant farmers from Germany and Sweden. In the mid-19th century, the Illinois and Michigan Canal and a sprawling railroad network facilitated trade, commerce, and settlement, making the state a transportation hub for the nation. By 1900, the growth of industrial jobs in the northern cities and coal mining in the central and southern areas attracted immigrants from Eastern and Southern Europe. Illinois became one of America's most industrialized states and remains a major manufacturing center. The Great Migration from the South established a large Black community, particularly in Chicago, which became a leading cultural, economic, and population center; its metropolitan area, informally referred to as Chicagoland, holds about 65% of the state's 12.8 million residents.

Two World Heritage Sites are in Illinois, the ancient Cahokia Mounds, and part of the Wright architecture site. A wide variety of protected areas seek to conserve Illinois' natural and cultural resources. Major centers of learning include the University of Chicago, University of Illinois, and Northwestern University. Three U.S. presidents have been elected while residents of Illinois: Abraham Lincoln, Ulysses S. Grant, and Barack Obama; additionally, Ronald Reagan was born and raised in the state. Illinois honors Lincoln with its official state slogan Land of Lincoln. The state is the site of the Abraham Lincoln Presidential Library and Museum in Springfield and the future home of the Barack Obama Presidential Center in Chicago.

Illinois has a highly diverse economy, with the global city of Chicago in the northeast, major industrial and agricultural hubs in the north and center, and natural resources such as coal, timber, and petroleum in the south. Owing to its central location and favorable geography, the state is a major transportation hub: the Port of Chicago has access to the Atlantic Ocean through the Great Lakes and Saint Lawrence Seaway and to the Gulf of Mexico from the Mississippi River via the Illinois Waterway. Chicago has been the nation's railroad hub since the 1860s, and its O'Hare International Airport has been among the world's busiest airports for decades. Illinois has long been considered a microcosm of the United States and a bellwether in American culture, exemplified by the phrase Will it play in Peoria?.