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.

DeKalb County is a county located in the U.S. state of Illinois. As of the 2020 United States census, the population was 100,420. Its county seat is Sycamore.