Whiteside County is a county located in the U.S. state of Illinois. According to the 2020 census, it had a population of 55,691. Its county seat is Morrison. The county is bounded on the west by the Mississippi River. Whiteside County comprises the Sterling, IL Micropolitan Statistical Area, which is also included in the Dixon-Sterling, IL Combined Statistical Area. U.S. President Ronald Reagan was born in 1911 in the Whiteside County community of Tampico.

Morrison is a city in Whiteside County, Illinois, United States. The population was 4,188 at the 2010 census, down from 4,447 in 2000. It is the county seat of Whiteside County. It is located on the Historic Lincoln Highway, the nation's first transcontinental highway and in Morrison was the site of two concrete "seedling miles", which served as prototypes of what an improved highway could do for the nation.