DeWitt County is a county located in the U.S. state of Illinois. As of the 2020 census, the population was 15,516. Its county seat is Clinton. The county was formed on March 1, 1839, from Macon and McLean counties. The county was named in honor of the seventh Governor of New York State, DeWitt Clinton.

Clinton is the largest city in DeWitt County, Illinois, United States. The population was 7,004 at the 2020 census. It is the county seat of DeWitt County.