Fernando Ruiz de Castro (d. Bayonne, 1377), nicknamed toda a lealdade de España ("of all the loyalty of Spain"), was a Galician nobleman of the House of Castro and prominent military figure. He was the third Count of Lemos, Trastámara and Sarria.
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 D. 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:
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: