Centralia is a city in Clinton, Jefferson, Marion, and Washington counties in the U.S. state of Illinois with the largest portion in Marion County. The city is the largest in three of the counties; Clinton, Marion, and Washington, but is not a county seat of any of them. The population was 12,182 as of the 2020 census, down from 13,032 in 2010.

Clinton County is located in the U.S. state of Illinois. At the 2020 census, the population was 36,899. Its county seat is Carlyle.