San Diego is a city in Duval and Jim Wells counties, Texas, United States. The population was 3,748 at the 2020 census and 4,488 at the 2010 …San Diego is a city in Duval and Jim Wells counties, Texas, United States. The population was 3,748 at the 2020 census and 4,488 at the 2010 census. It is located primarily in Duval County, of which it is the county seat.