Quincy, known as Illinois's "Gem City", is a city in and the county seat of Adams County, Illinois, United States, located on the Mississippi River. The 2020 census counted a population of 39,463 in the city itself, down from 40,633 in 2010. As of July 1, 2015, the Quincy Micro Area had an estimated population of 77,220. During the 19th century, Quincy was a thriving transportation center as riverboats and rail service linked the city to many destinations west and along the river.