The Illinois Campaign, also known as Clark's Northwestern Campaign (1778-1779), is a series of events during the American Revolutionary War in which a small force of Virginia militiamen, led by George Rogers Clark, seizes control of several British posts in the Illinois Country, in what are now Illinois and Indiana in the Midwestern United States.

The campaign is the best-known action of the western theater of the war and the source of Clark's reputation as an early American military hero.

In July 1778, Clark and his men cross the Ohio River from Kentucky and take control of Kaskaskia, Vincennes, and several other villages in British territory.

The occupation is accomplished without firing a shot because most of the Canadien and Native American inhabitants, who peacefully co-exist with one another, are unwilling to take up arms on behalf of the British Empire.

To counter Clark's advance, Henry Hamilton, the British lieutenant governor at Fort Detroit, reoccupies Vincennes with a small force

In February 1779, Clark returns to Vincennes in a surprise winter expedition and retakes the town, capturing Hamilton in the process.

Virginia capitalizes on Clark's success by establishing the region as Illinois County, Virginia.

The importance of the Illinois campaign has been the subject of much debate.

Because the British ceded the entire Northwest Territory to the United States in the 1783 Treaty of Paris, some historians have credited Clark with nearly doubling the size of the original Thirteen Colonies by seizing control of the Illinois Country during the war.

For this reason, Clark was nicknamed the "Conqueror of the Northwest", and his Illinois campaign—particularly the surprise march to Vincennes—was greatly celebrated and romanticized.

Other historians have downplayed the importance of the campaign, arguing that Clark's "conquest" was a temporary occupation that had no impact on the boundary negotiations in Europe.