Seneca is a village in LaSalle and Grundy counties in the U.S. state of Illinois. The population was 2,353 at the 2020 census, down from 2,371 at the 2010 census. The LaSalle County portion of Seneca is part of the Ottawa Micropolitan Statistical Area, while the small portion that lies in Grundy County is part of the Chicago–Naperville–Joliet Metropolitan Statistical Area.