Chicago[a] 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 2.74 million at the 2020 census,[9] 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.