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 "Elderedo" or “Elder-Redo” (depending on the source)—a combination of the last names of the town's two founders, Judge Samuel Elder, his son William, grandson Francis Marion, and neighbor Joseph Reed, 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.