Eldorado is a city in Saline County, Illinois, United States. The population was 4,122 at the 2010 census, with a 1980 peak of 5,198. Although the city's name is spelled as if it were Spanish, the name was originally "Eldereado" or “Elder-Reado” —a combination of the last names of the town's two founders, Judge Samuel Elder, his son William, grandson Francis Marion, and neighbor Joseph Read, and his brother William. According to legend, a signpainter for the railroad painted the name "Eldorado" on the train depot; as a result, the spelling and pronunciation (el-do-RAY-doh) was forever changed.

Saline County is a county in Southern Illinois. At the 2020 census, it had a population of 23,768. The largest city and county seat is Harrisburg. This area of Southern Illinois is known locally as "Little Egypt".