The Illinois-Wabash Company, formally known as the United Illinois and Wabash Land Company, was a company formed in 1779 from the merger of the Illinois Company and the Wabash Company. The two companies had been established in order to purchase land from Native Americans in the Illinois Country, a region of North America acquired by Great Britain in 1763. The Illinois Company purchased two large tracts of land in 1773; the Wabash Company purchased two additional tracts in 1775.
Because the Royal Proclamation of 1763 forbade private purchase of Native American lands, Great Britain refused to recognize these transactions. Following the outbreak of the American Revolutionary War, officials of the merged Illinois-Wabash Company appealed to both Virginia (which claimed the Illinois Country) and to the United States to recognize their land purchases but were unsuccessful. After the United States bought the land in question from Native Americans and resold it, the matter eventually went to the Supreme Court of the United States. In Johnson v. McIntosh (1823), the Court ruled that the U.S. government, following earlier British precedent, would not recognize private purchases of native lands, and that Illinois-Wabash Company's purchases were therefore invalid.