Illinois (/ˌɪlɪˈnɔɪ/ ^ⓘ IL-ih-NOY) 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 rivers 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, Sweden and elsewhere. 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, especially in the city of Chicago, which became the world's fastest growing city by the late 19th century. 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, though he served for California. 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?.

"Illinois" is the modern spelling for the early French Catholic missionaries and explorers' name for the Illinois Native Americans, a name that was spelled in many different ways in the early records.

American scholars previously thought the name Illinois meant 'man' or 'men' in the Miami-Illinois language, with the original iliniwek transformed via French into Illinois. This etymology is not supported by the Illinois language,^{[citation needed]} as the word for "man" is ireniwa, and plural of "man" is ireniwaki. The name Illiniwek has also been said to mean 'tribe of superior men',^{[citation needed]} which is a false etymology. The name Illinois derives from the Miami-Illinois verb irenwe·wa 'he speaks the regular way'. This was taken into the Ojibwe language, perhaps in the Ottawa dialect, and modified into ilinwe· (pluralized as ilinwe·k). The French borrowed these forms, spelling the /we/ ending as -ois, a transliteration of that sound in the French of that time. The current spelling form, Illinois, began to appear in the early 1670s, when French colonists had settled in the western area. The Illinois's name for themselves, as attested in all three of the French missionary-period dictionaries of Illinois, was Inoka, of unknown meaning and unrelated to the other terms.

American Indians of successive cultures lived along the waterways of the Illinois area for thousands of years before the arrival of Europeans. The Koster Site has been excavated and demonstrates 7,000 years of continuous habitation. Cahokia, the largest regional chiefdom and Urban Center of the Pre-Columbian Mississippian culture, was located near present-day Collinsville, Illinois. They built an urban complex of more than 100 platform and burial mounds, a 50-acre (20 ha) plaza larger than 35 football fields, and a woodhenge of sacred cedar, all in a planned design expressing the culture's cosmology. Monks Mound, the center of the site, is the largest Pre-Columbian structure north of the Valley of Mexico. It is 100 ft (30 m) high, 951 ft (290 m) long, 836 ft (255 m) wide, and covers 13.8 acres (5.6 ha). It contains about 814,000 cu yd (622,000 m³) of earth. It was topped by a structure thought to have measured about 105 ft (32 m) in length and 48 ft (15 m) in width, covered an area 5,000 sq ft (460 m²), and been as much as 50 ft (15 m) high, making its peak 150 ft (46 m) above the level of the plaza. The finely crafted ornaments and tools recovered by archaeologists at Cahokia include elaborate ceramics, finely sculptured stonework, carefully embossed and engraved copper and mica sheets, and one funeral blanket for an important chief fashioned from 20,000 shell beads. These artifacts indicate that Cahokia was truly an urban center, with clustered housing, markets, and specialists in toolmaking, hide dressing, potting, jewelry making, shell engraving, weaving and salt making.

The civilization vanished in the 15th century for unknown reasons, but historians and archeologists have speculated that the people depleted the area of resources. Many indigenous tribes engaged in constant warfare. According to Suzanne Austin Alchon, "At one site in the central Illinois River valley, one third of all adults died as a result of violent injuries." The next major power in the region was the Illinois Confederation or Illini, a political alliance. Around the time of European contact in 1673, the Illinois confederation had an estimated population of over 10,000 people. As the Illini declined during the Beaver Wars era, members of the Algonquian-speaking Potawatomi, Miami, Sauk, and other tribes including the Fox (Meskwaki), Iowa, Kickapoo, Mascouten, Piankeshaw, Shawnee, Wea, and Winnebago (Ho-Chunk) came into the area from the east and north around the Great Lakes.