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.

LaSalle County is located within the Fox Valley and Illinois River Valley regions of the U.S. state of Illinois. As of the 2020 Census, it had a population of 109,658. Its county seat and largest city is Ottawa. LaSalle County is part of the Ottawa, IL Micropolitan Statistical Area of Northern Illinois.