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is a city in Morgan County, Illinois, United States. The population was 18,940 at the 2000 census. It is the county seat of Morgan County.
The town was named in 1825 for future president Andrew Jackson, the commander of American forces at the Battle of New Orleans (1815) and presidential hopeful in 1824. Jacksonville was a major stopping point on the historic Underground Railroad. An Annual Civil War reenactment celebration is named for Jacksonville resident U.S. Army General Benjamin Grierson.
Jacksonville is the principal city of the Jacksonville Micropolitan Statistical Area, which includes all of Morgan and Scott counties.
According to the United States Census Bureau, the city has a total area of 26.8 km² (10.3 mi²). 26.2 km² (10.1 mi²) of it is land and 0.5 km² (0.2 mi²) of it (1.84%) is water.