Castro is a Romance language word that originally derived from Latin castrum, a pre-Roman military camp or fortification (cf: Greek: kastron; Proto-Celtic:*Kassrik; ). The English-language equivalent is chester.

Don Víctor Ramón Castro (February 2, 1820 – May 5, 1900) was a Californio ranchero, politician, and businessman. He was one of the largest landowners in Contra Costa and served as a Contra Costa County Supervisor.

Américo Castro y Quesada (May 4, 1885 – July 25, 1972) was a Spanish cultural historian, philologist, and literary critic who challenged some of the prevailing notions of Spanish identity, raising controversy with his conclusions that Spaniards did not become the distinct group that they are today until after the Islamic conquest of Hispania of 711, an event that turned them into an Iberian caste co-existing among Moors and Jews, and that the history of Spain and Portugal was adversely affected with the success in the 11th to the 15th centuries of the "Reconquista" or Christian reconquest of the Iberian Peninsula and with the Spanish expulsion of the Jews (1492).