The Illinois Country (French: Pays des Illinois), also known as Upper Louisiana, is a region in what is now the Midwestern United States that is explored and settled by the French during the 17th and 18th centuries.

The terms refer to the entire Upper Mississippi River watershed, though settlement is concentrated in what are now the U.S. states of Illinois, Missouri, and Indiana.

First explored in 1673 by the expedition of Louis Joliet and Jacques Marquette, the area is soon claimed by France and becomes part of Louisiana, together with Lower Louisiana.Up until 1717, the Illinois Country is governed by the French province of Canada, but by order of King Louis XV, the Illinois Country is annexed to the French province of Louisiana, with the northern border being the Illinois River.

The territory thus becomes known as "Upper Louisiana".

By the mid-18th century, the major settlements include 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 Missouri.

The region is initially governed from Canada, but by the 1720s a local governmental infrastructure is in place, in which the local towns report to the commandant of Fort de Chartres, who in turn reports to the governor general of Louisiana in New Orleans.

Upper and Lower Louisiana east of the Mississippi River are ceded to the British in 1763, and the west to the Spanish, after the French and Indian War.

Some Canadien settlers remain in the area, while others cross the Mississippi, forming new settlements such as St. Louis.

The eastern part of the region becomes part of the Province of Quebec.

However, since its inhabitants have been under the authority of Louisiana for some time, the inhabitants choose to side with the Americans during the American Revolutionary War.

Eventually, the western side of the Mississippi is sold to the United States by Napoleon in 1803.

French Canadian language and culture continue to exist in the area, with the Missouri French dialect still being spoken into the 20th century.Because of the deforestation that results from the cutting of much wood for fuel during the 19th-century age of steamboats, the Mississippi River becomes more shallow and broad, with more severe flooding and lateral changes in its channel in the stretch from St. Louis to the confluence with the Ohio River.

As a consequence, many architectural and archeological resources are lost to flooding and destruction of early French colonial villages originally located near the river, such as Kaskaskia, St. Philippe, and Cahokia, Illinois, and old Ste.

Genevieve, Missouri.