Juan de Borja y Castro was a nobleman and diplomat; he was the third son of San Francisco de Borja y Aragón, IV Duke of Gandía. He was the dedicatee of the manuscript B-Bc 27086, compiled in Vienna between 1577 and 1579, while Juan de Borja, I Count of Mayalde, served as mayordomo mayor of the Empress Dowager Maria of Austria, Philip II's sister and widow of Maximillian II. Juan de Borja was the oncle of the Duke of Lerma.