Williamson is a village in Madison County, Illinois, United States. The population was 230 at the 2010 census.
Williamson had its start as a coal town. The village is named for the land's original owners, John and Matthew Williamson.
Williamson is located at 38°59′15″N 89°45′51″W (38.987418, -89.764039).
According to the 2010 census, Williamson has a total area of 1.556 square miles (4.03 km2), of which 1.51 square miles (3.91 km2) (or 97.04%) is land and 0.046 square miles (0.12 km2) (or 2.96%) is water.
As of the census of 2000, there were 251 people, 99 households, and 66 families residing in the village. The population density was 165.4 people per square mile (63.8/km2). There were 112 housing units at an average density of 73.8 per square mile (28.4/km2). The racial makeup of the village was 97.61% White and 2.39% Native American.
There were 99 households, out of which 33.3% had children under the age of 18 living with them, 51.5% were married couples living together, 15.2% had a female householder with no husband present, and 33.3% were non-families. 26.3% of all households were made up of individuals, and 18.2% had someone living alone who was 65 years of age or older. The average household size was 2.54 and the average family size was 3.08.
In the village, the population was spread out, with 26.7% under the age of 18, 7.6% from 18 to 24, 33.1% from 25 to 44, 18.3% from 45 to 64, and 14.3% who were 65 years of age or older.