The Illinois Country (French: Pays des Illinois [pɛ.i dez‿i.li.nwa]; lit. 'land of the Illinois people'; Spanish: País de los ilinueses), also referred to as Upper Louisiana (French: Haute-Louisiane [ot.lwi.zjan]; Spanish: Alta Luisiana), was a vast region of New France claimed in the 1600s that later fell under Spanish and British control before becoming what is now part of the Midwestern United States. While the area claimed included the entire Upper Mississippi River watershed, French colonial settlement was concentrated along the Mississippi and Illinois Rivers in what is now the U.S. states of Illinois and Missouri, with outposts on the Wabash River in Indiana. Explored in 1673 from Green Bay to the Arkansas River by the Canadien expedition of Louis Jolliet and Jacques Marquette, the area was claimed by France. It was settled primarily from the Pays d'en Haut in the context of the fur trade, and in the establishment of missions from Canada by French Catholic religious orders. Over time, the fur trade took some French to the far reaches of the Rocky Mountains, especially along the branches of the broad Missouri River valley. The "Illinois" in the territory's name is a reference to the Illinois Confederation, a group of related Algonquian native peoples.

The Illinois Country was governed from the French province of Canada until 1717 when, by order of King Louis XV, it was annexed to the French province of Louisiana, becoming known as "Upper Louisiana". By the mid-18th century, major settlements included Cahokia, Kaskaskia, Chartres, Saint Philippe, and Prairie du Rocher, all on the east side of the Mississippi in present-day Illinois; and Ste. Genevieve across the river in what is now Missouri, as well as Fort Vincennes in what is now Indiana.

As a consequence of the French defeat in the French and Indian War in 1764, the Illinois Country east of the Mississippi River was ceded to the British and became part of the British Province of Quebec; the land west of the river was ceded to Spanish Louisiana.

During the American Revolutionary War, Virginian George Rogers Clark led the Illinois campaign against the British. Illinois Country east of the Mississippi River along with what was then much of Ohio Country became part of Illinois County, Virginia, claimed by right of conquest. The county was abolished in 1782. In 1784, Virginia ceded its claims to the U.S. government and the area was incorporated as part of the Northwest Territory. The name lived on as Illinois Territory between 1809 and 1818, and as the State of Illinois after its admission to the union in 1818. The Spanish-occupied portion of Illinois Country west of the Mississippi was acquired by the United States from France in the Louisiana Purchase of 1803.

The boundaries of the Illinois Country were defined in a variety of ways, but the region now known as the American Bottom was nearly at the center of all descriptions. One of the earliest known geographic features designated as Ilinois was what later became known as Lake Michigan, on a map prepared in 1671 by French Jesuits. Early French missionaries and traders referred to the area southwest and southeast of the lake, including much of the upper Mississippi Valley, by this name. Illinois was also the name given to an area inhabited by the Illiniwek. A map of 1685 labels a large area southwest of the lake les Ilinois; in 1688, the Italian cartographer Vincenzo Coronelli labeled the region (in Italian) as Illinois country. In 1721, the seventh military district of Louisiana was named Illinois. It included more than half of the present state, as well as the land between the Arkansas River and the line of 43 degrees north latitude, between the Rocky Mountains and the Mississippi River. A royal ordinance of 1722—following the transfer of the Illinois Country's governance from Canada to Louisiana—may have featured the broadest definition of the region, making it coterminous with Upper Louisiana: all land claimed by France south of the Great Lakes and north of the mouth of the Ohio River, including both banks of the Mississippi as well as the lower Missouri Valley. In 1723, the area around the Wabash River became a separate district.

A generation later, trade conflicts between Canada and Louisiana led to a more defined boundary between the French colonies; in 1745, Louisiana governor general Vaudreuil set the northeastern bounds of his domain as the Wabash valley up to the mouth of the Vermilion River (near present-day Danville, Illinois); from there, northwest to le Rocher on the Illinois River, and from there west to the mouth of the Rock River (at present-day Rock Island, Illinois). Thus, Vincennes and Peoria were near the limit of Louisiana's reach; the outposts at Ouiatenon (on the upper Wabash near present-day Lafayette, Indiana), Checagou, Fort Miamis (near present-day Fort Wayne, Indiana) and Prairie du Chien operated as dependencies of Canada.

This boundary between Canada and the Illinois Country remained in effect until the Treaty of Paris in 1763, after which France surrendered its remaining territory east of the Mississippi to Great Britain. (Although British forces had occupied the "Canadian" posts in the Illinois and Wabash countries in 1761, they did not occupy Vincennes or the Mississippi River settlements at Cahokia and Kaskaskia until 1764, after the ratification of the peace treaty.) As part of a general report on conditions in the newly conquered lands, Gen. Thomas Gage, then commandant at Montreal, explained in 1762 that, although the boundary between Louisiana and Canada was not exact, it was understood that the upper Mississippi above the mouth of the Illinois was in Canadian trading territory.

Distinctions became somewhat clearer after the Treaty of Paris in 1763, when Britain acquired Canada and the land claimed by France east of the Mississippi and Spain acquired Louisiana west of the Mississippi. Many French settlers moved west across the river to escape British control. On the west bank, the Spanish also continued to refer to the western region governed from St. Louis as the District of Illinois and referred to St. Louis as the city of Illinois.