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.