Rock Island is a city in and the county seat of Rock Island County, Illinois, United States. The original Rock Island, from which the city name is derived, is now called Arsenal Island. The population was 37,108 at the 2020 census. Located on the Mississippi River, it is one of the Quad Cities, along with neighboring Moline, East Moline, and the Iowa cities of Davenport and Bettendorf. The Quad Cities has a population of about 480,000.