Bostonia is a census-designated place (CDP) in San Diego County, California. A neighborhood comprising areas within the city limits of northeast El Cajon, as well as areas of unincorporated San Diego County bears the same name. The population of the CDP was 15,379 at the 2010 census, up from 15,169 at the 2000 census (these figures do not include people living within the city of El Cajon).