Fernando Sánchez de Castro (1241–1275) was an Aragonese infante (royal prince), crusader and rebel leader.
Fernando was an illegitimate son of King James I of Aragon by his mistress Blanca, daughter of Sancho de Antillón. Their relationship can be dated to 1241, when James gave her the castle of Castro. Fernando later took the title of baron of Castro.
In April 1261, Fernando and Guillem de Montgrí were sent to Naples as ambassadors to King Manfred of Sicily. As a result of this embassy, James's heir, the infante Peter, was married to Manfred's daughter, Constance. Fernando was a witness to their wedding on 13 June 1262 in Montpellier.
At the cortes of 1264 in Zaragoza, Fernando clashed with his father over the extension of the bovatge tax on livestock from Catalonia to Aragon. When the Aragonese nobility withdrew from the cortes, Fernando sided with them. He thereby gained enduring popularity with the nobility of Aragon and married a daughter of Ximeno II d'Urrea. Despite these clashes, Fernando remained a favourite of his father.
Fernando took part in the crusade to the Holy Land launched by his father on 4 September 1269. When his father diverted the fleet to Aigues-Mortes after a storm and opted to abandon the enterprise, Fernando and his half-brother, Pedro Fernández de Híjar, continued on to Acre. They and their followers were too few, however, to accomplish anything there. He may have made contact with King Charles I of Sicily during his return voyage from Acre.
In 1270 or 1271, Fernando was in exile in Montpellier when Charles I offered to pay him 8,000 livres tournois and transport costs if he served him for a year in the defence of Trapani. He was to bring forty knights, four squires and twenty mounted crossbowmen, but he did not accept the offer.
In February 1272, James convoked a cortes in Lleida for the following the month to resolve the differences between Fernando and the infante Peter, but the only result was a break between the king and his heir, who accused Fernando of seeking to dethrone their father. In December 1273, James and Peter were reconciled, and the latter pledged not to harm Fernando.
