Assumption is a city in Christian County, Illinois, United States. The population was 1,168 at the 2010 census.
According to the 2010 census, Assumption has a total area of 0.88 square miles (2.28 km), all land.
As of the census of 2000, there were 1,261 people, 551 households, and 353 families residing in the city. The population density was 1,433.1 people per square mile (553.3/km²). There were 607 housing units at an average density of 689.9 per square mile (266.3/km²). The racial makeup of the city was 99.84% White, 0.08% Asian, and 0.08% from two or more races. Hispanic or Latino of any race were 0.16% of the population.
There were 551 households out of which 28.7% had children under the age of 18 living with them, 51.4% were married couples living together, 8.9% had a female householder with no husband present, and 35.8% were non-families. 33.2% of all households were made up of individuals and 18.9% had someone living alone who was 65 years of age or older. The average household size was 2.29 and the average family size was 2.86.
In the town the population was spread out with 25.6% under the age of 18, 7.0% from 18 to 24, 27.0% from 25 to 44, 21.1% from 45 to 64, and 19.3% who were 65 years of age or older. The median age was 38 years. For every 100 females, there were 82.8 males. For every 100 females age 18 and over, there were 81.4 males.
The median income for a household in the town was $34,474, and the median income for a family was $41,417. Males had a median income of $35,650 versus $17,292 for females. The per capita income for the city was $16,421. About 5.2% of families and 9.5% of the population were below the poverty line, including 11.6% of those under age 18 and 11.7% of those age 65 or over.