Thebes is a village in and the former county seat of Alexander County, Illinois, United States. The population was 208 at the 2020 census, down from 436 at the 2010 census. It is part of the Cape Girardeau–Jackson, MO-IL Metropolitan Statistical Area. In 1860 the county seat was moved to Cairo, Illinois, at the confluence of the Ohio and Mississippi rivers.

Alexander County is the southernmost and southwesternmost county in the U.S. state of Illinois. As of the 2020 census, the population was 5,240. Its county seat is Cairo and its western boundary is formed by the Mississippi River.