Fernando Ruiz de Castro (d. Bayonne, 1377), was a Galician nobleman of the House of Castro and prominent military figure. He was the third Count of Lemos, Trastámara and Sarria. He is often referred to by the appellation "Toda la lealtad de España" ("All the loyalty of Spain"), from an inscription on his tomb in Bayonne.
He was the son of Pedro Fernández de Castro and Isabel Ponce de Leon. He was the brother of Juana de Castro, wife of King Peter I of Castile and half-brother of the controversial Inês de Castro (mistress of King Peter I of Portugal), and Álvaro Pires de Castro (leader of the Portuguese branch of the House of Castro).
Fernando Ruiz de Castro became a prominent figure in the circle of King Peter of Castile, and served as lord steward and standard-bearer of the realm. During the Castilian Civil War, he was the leading partisan of Peter's camp, and continued fighting for the Petrist cause against the House of Trastámara even after the king's death in 1369.
His first marriage in 1354 to Juana Alfonso, illegitimate daughter of Alfonso XI of Castile, produced two children:
- Pedro de Castro - he was exiled and died in England.
- Guterre de Castro
Divorced from his first wife, in 1367 he married Eleanor Enriquez, Lady of Melgar, widow of Alonso de Guzmán and daughter of Enrique Enriquez and his wife, Urraca Ponce de Leon. This marriage produced no offspring.
From his mistress Mília Gonçalves, Lady of Assequins, he had one illegitimate son:
- Eduardo Pardo de Guevara y Valdes (1983) "La nobleza gallega y la entronizacion de los Trastamara en Castilla" (p.286); (1985) "Dona Isabel de Castro: Apuntes criticos sobre su discutida filiacon", XXV Anos de la Escuela de Genealogia, Heraldica y Nobiliaria, Madrid (p.421).
- J. Salazar y Acha (2008) "La Nobleza Titulada Medieval en la Corona de Castilla", Anales de la Real Academia Matritense de Heraldica y Genalogia p.15
- C. O. Serrano (2005) Beatriz de Portugal: la pugna dinástica Avís-Trastámara, La Coruña: Lugami. p.65
- The tomb is now lost, but record of the inscription is found in, e.g. 1588 ed. of Nobleza del Andalusia of Gonzalo Argote de Molina; 1657 ed. of Excellencias y Primacias del Apostol Santiago by Geronimo Pardo, 1780 ed. Cronicas de los reyes de Castilla by Pedro Lopes de Ayala. Some historians (e.g. Victoria Ancheta (1969) Galiza Feudal, p.351) believe the appellation was invented much later, or might be a misreading of the original.