Old Jamestown is an unincorporated community and census-designated place (CDP) in St. Louis County, Missouri, United States. The population was 19,790 at the 2020 census. Old Jamestown is northwest of the city of St. Louis and borders the Missouri River.
According to the United States Census Bureau, the CDP has a total area of , of which , or 0.06%, is water. The community is located mostly to the north and west of U.S. Route 67 in northern St. Louis County. West Alton is to the north, across the Missouri River. Florissant is to the southwest, and Black Jack and Spanish Lake are to the south. The CDP is located north of downtown St. Louis.
Old Jamestown first appeared as a census designated place in the 2010 U.S. census.
As of the 2020 census, Old Jamestown had a population of 19,790. The median age was 47.6 years. 19.1% of residents were under the age of 18 and 21.3% of residents were 65 years of age or older. For every 100 females there were 89.2 males, and for every 100 females age 18 and over there were 86.5 males age 18 and over.
92.2% of residents lived in urban areas, while 7.8% lived in rural areas.
There were 7,570 households in Old Jamestown, of which 26.5% had children under the age of 18 living in them. Of all households, 51.8% were married-couple households, 13.9% were households with a male householder and no spouse or partner present, and 29.8% were households with a female householder and no spouse or partner present. About 23.1% of all households were made up of individuals and 10.4% had someone living alone who was 65 years of age or older.
There were 7,925 housing units, of which 4.5% were vacant. The homeowner vacancy rate was 1.1% and the rental vacancy rate was 13.1%.
The vast majority of the CDP is in the Hazelwood School District. A small portion is in the Ferguson-Florissant School District.