Effingham County is a county located in the south central part of the U.S. state Illinois. As of the 2020 census, the population was 34,668. Its county seat and largest city is Effingham. Some other cities in Effingham County, Illinois include Altamont, Teutopolis, Beecher City, Montrose, Dieterich, Shumway, Watson, Mason, and Edgewood. Effingham County comprises the Effingham, IL Micropolitan Statistical Area.