San Dimas is a city in the San Gabriel Valley of Los Angeles County, California, United States. At the 2020 census, its population was 34,9…San Dimas is a city in the San Gabriel Valley of Los Angeles County, California, United States. At the 2020 census, its population was 34,924. It historically took its name from San Dimas Canyon in the San Gabriel Mountains above the northern section of present-day San Dimas.