Bostonia is a neighborhood in San Diego County, California, comprising part of the northeastern portion of the city of El Cajon, as well as adjacent unincorporated areas of San Diego County. The portion of Bostonia that lies outside the El Cajon city limits is classified as a census-designated place for statistical purposes by the United States Census Bureau. The population of the CDP was 15,379 at the 2010 census, up from 15,169 at the 2000 census.