Cortland is an incorporated town in DeKalb County, Illinois, United States. The population was 4,398 at the 2020 Census. It was previously 4,270 at the 2010 census, up from 2,066 in 2000. Because of its status as an incorporated town, Cortland has adopted the slogan, "The Third Largest Town in Illinois". Only the towns of Cicero and Normal are larger, in terms of population. Most municipalities in Illinois are defined as "cities" or "villages."