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.

Ottawa is a city in and the county seat of LaSalle County, Illinois, United States. It is located at the confluence of the navigable Fox River and Illinois River, the latter being a conduit for river barges and connects Lake Michigan at Chicago, to the Mississippi River, and North America's 25,000 mile river system. The population estimate was 18,668, as of 2022. It is the principal city of the Ottawa, IL Micropolitan Statistical Area.