Affton is an unincorporated community and census-designated place (CDP) in south St. Louis County, Missouri, United States, near St. Louis.

[4] According to the United States Census Bureau, the community has a total area of 4.6 square miles (12 km2), all land.

[6] As of the census[3] of 2000, there were 20,535 people, 8,892 households, and 5,655 families residing in the CDP.

There were 9,128 housing units at an average density of 1,991.5 per square mile (768.9/km2).

31.9% of all households were made up of individuals, and 14.0% had someone living alone who was 65 years of age or older.