Chicago is the most populous city in the U.S. state of Illinois and in the Midwestern United States. Located on the western shore of Lake Mic…Chicago is the most populous city in the U.S. state of Illinois and in the Midwestern United States. Located on the western shore of Lake Michigan, it is the third-most populous city in the United States with a population of 2,746,388 at the 2020 census, while the Chicago metropolitan area has 9.41 million residents and is the third-largest metropolitan area in the nation. Chicago is the seat of Cook County, the second-most populous county in the U.S.