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.

Cairo is the southernmost city in Illinois and the county seat of Alexander County. A river city, Cairo has the lowest elevation of any location in Illinois and is the only Illinois city to be surrounded by levees. It is in the river-crossed area of Southern Illinois known as "Little Egypt", for which the city is named, after Egypt's capital on the Nile. The city is coterminous with Cairo Precinct.