Mattoon is a city in Coles County, Illinois, United States. The population was 18,555 as of the 2010 census. It is a principal city of the Charleston–Mattoon Micropolitan Statistical Area.
Mattoon was the site of the "Mad Gasser" attacks of the 1940s.
Early settlers from the South lived in forested areas along the headwaters of the Little Wabash River to the southwest of the present town. They distrusted the prairie, which they saw as the source of fevers.