Somonauk is a village in DeKalb County and LaSalle counties in the U.S. state of Illinois. The population was 1,295 at the 2000 census. A special census in 2008 set the population at 1,832.
The DeKalb County portion of Somonauk is part of the Chicago–Naperville–Joliet Metropolitan Statistical Area, while the small portion that lies in LaSalle County is part of the Ottawa–Streator Micropolitan Statistical Area.
Somonauk is located at 41°38′2″N 88°40′55″W / 41.63389°N 88.68194°W / 41.63389; -88.68194 (41.633940, -88.681945). The village is situated along the boundary between DeKalb and LaSalle counties. In the 2000 census, 1,190 of Somonauk's 1,295 residents (91.9%) lived in DeKalb County and 105 (8.1%) lived in LaSalle County.