Centralia is a city in Clinton, Jefferson, Marion, and Washington counties in the U.S. state of Illinois with the largest portion being within 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 13,032 as of the 2010 census, down from 14,136 in 2000.
Centralia is named for the Illinois Central Railroad, built in 1853. The city was founded at the location where the two original branches of the railroad converged. Centralia was first chartered as a city in 1859.
In the southern city limits is the intersection of the Third Principal Meridian and its baseline. This initial point was established in 1815, and it governs land surveys for about 60% of the state of Illinois, including Chicago. The original monument is at the junction of Highway 51 and the Marion-Jefferson County Line Road; today there is a small easement situated in the northeast corner of this intersection, which contains a monument and historic marker.
Production of the PayDay candy bar began here in 1938. Michael Moore's documentary, The Big One (1998), opens with the closing of this candy bar plant in the late 20th century.