Somonauk is a village in DeKalb and LaSalle Counties in the U.S. state of Illinois. The population was 1,893 at the 2010 Census.The DeKalb County portion of Somonauk is part of the Chicago-Naperville-Elgin, IL-IN-WI Metropolitan Statistical Area, while the small portion that lies in LaSalle County is part of the Ottawa–Peru, IL Micropolitan Statistical Area.