Quincy (/ˈkwɪnsi/ KWIN-see) is a city in Adams County, Illinois, United States, and its county seat. 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. It was Illinois' second-largest city, surpassing Peoria in 1870. The city has several historic districts, including the Downtown Quincy Historic District and the South Side German Historic District, which display the architecture of Quincy's many German immigrants from the late 19th century.

Quincy's location along the Mississippi River has attracted settlers for centuries. The French became the first European presence to colonize the region, after Louis Jolliet, Jacques Marquette, Jamison Knapp and the La Salle Expeditions explored the Upper Mississippi River Valley. Fur goods became a valuable commodity of the region, and European explorers and merchants alike were attracted to the prospects of the growing fur trade of the North American frontier. The Mississippi River, acting as a superhighway for transporting goods downstream, became the area's most vital transportation asset.

Following the events of the Seven Years' War, which ended in 1763, Great Britain took control of New France, and the area that is now Illinois became part of the Indian Reserve. After the American Revolutionary War a few decades later, the area became part of the Northwest Territory of the United States, and eventually the State of Illinois in 1818.

After the War of 1812, the American government granted military tracts to veterans as a means to help populate the West. Peter Flinn, having acquired the land from veteran Mark McGowan for his military service in 1819, ended up selling 160 acres (0.65 km²) of land acquisitions to Moravia, New York native John Wood for $60 (~$1,739 in 2024). John Wood later founded Quincy, which at the time was coined Bluffs, Illinois.

In 1825, Bluffs renamed their community Quincy and became the seat of government for Adams County, both named after newly elected President John Quincy Adams. In addition, they originally named the town square John Square until eventually changing its name to Washington Square. In December 1830, Peter Felt, who had recently arrived with his family from New Hampshire, and others held the first service at what would become the first congregational churches in the state. In June 1834, Quincy was incorporated, with Archibald Williams, Joseph T. Holmes, S. W. Rogers, Levi Wells, and Michael Mast elected as trustees. Quincy incorporated as a city in 1840, with Ebenezer Moore elected as its first mayor.

In 1837, following the signing of Missouri Executive Order 44, which called for the expulsion of Mormons from Missouri and the extermination of those who refused, many members of the Church of Jesus Christ of Latter-day Saints fled persecution in Missouri and found shelter in Quincy. Despite being vastly outnumbered by Mormon refugees, residents provided food and lodging for the displaced people. Joseph Smith then led members of the Church of Jesus Christ of Latter-day Saints 40 miles (64 km) upstream to Nauvoo, Illinois, in hopes of finding a permanent home. Also in 1838, Quincy sheltered the Potawatomi tribe as they were forcibly relocated from Indiana to Kansas.

The 1850s and 1860s brought increased prosperity to Quincy. Steamboats and railroads began linking Quincy to places west, making the city a frequent destination for immigrants. The founding of the Chicago, Burlington and Quincy Railroad in 1855, and the construction of the Quincy Rail Bridge, were major drivers for creating a transportation hub in the region to further commerce. It is during this time that the city's population grew enormously, from just under 7,000 residents in 1850 to 24,000 by 1870, helping Quincy surpass Peoria in becoming the second-largest city in the state (at that time).