Quincy is a city in and the county seat of Adams County, Illinois, United States, located on the Mississippi River. The population was 39,463 as of the 2020 census, down from 40,633 in 2010. The Quincy micropolitan area had 114,649 residents. Known as Illinois's "Gem City", Quincy was a thriving transportation center as riverboats and rail service linked the city to many destinations west and along the river during the 19th century.