St. John or Saint John[2] is a city in St. Louis County, Missouri, United States, with its commercial district centered along St. Charles Rock Road.

St. John was once a sundown town, a locality that forbade people of color.

It once had a sign on its main street reading, "Negro, don't let the sun set on your head here.

"[4] According to the United States Census Bureau, the city has a total area of 1.42 square miles (3.68 km2), all land.

There were 2,953 housing units at an average density of 2,079.6 per square mile (802.9/km2).

30.0% of all households were made up of individuals, and 8.8% had someone living alone who was 65 years of age or older.

29.2% of all households were made up of individuals, and 10.7% had someone living alone who was 65 years of age or older.