DeKalb is a city in DeKalb County, Illinois, United States. The population was 43,862 according to the 2010 census, up from 39,018 at the 2000 census. The city is named after decorated Franconian-French war hero Johann de Kalb, who died during the American Revolutionary War.Founded in 1856, DeKalb became important in the development and manufacture of barbed wire, especially for agriculture and raising livestock.