Effingham County is a county in the U.S. state of Illinois. As of the 2010 census, the population was 34,242. Its county seat and largest city is Effingham. Some other cities in Effingham County, Illinois include Altamont, Teutopolis (T-Town), Beecher City, Montrose, Dieterich, Shumway, Watson, Mason, Edgewood, and Funkhouser.