Ferdinand I (c. 1015 – 24 December[1] 1065), called the Great (el Magno), was the count of Castile from his uncle's death in 1029 and the king of León after defeating his brother-in-law in 1037.

While Ferdinand inaugurated the rule of the Navarrese Jiménez dynasty over western Spain, his rise to preeminence among the Christian rulers of the peninsula shifted the focus of power and culture westward after more than a century of Leonese decline.

[5] One charter from Pamplona, dated 29 September 1023, is witnessed by Sancho's mother, Jimena Fernández, his wife Muniadona, her children, listed García, Ferdinand then Gonzalo, and their brother, the illegitimate Ramiro.

[13] The first indication that Ferdinand was independently reigning over Castile, or was at least recognised as count in his own right, is a charter of 1 November 1032 from the monastery of San Pedro de Arlanza, which does not mention his father, but dates it to the time of "Fernando Sánchez bearing the county".

The only sovereign whose regnal year was used was Sancho III, making Ferdinand the first count of Castile not to recognise the suzerainty of the king of León.

[17] In a dispute over the territory between the Cea and Pisuerga, Ferdinand, nominally a vassal of Bermudo III, defeated and killed his suzerain at the Battle of Tamarón on 4 September 1037.

Although Navarre at that time included the traditionally Castilian lands of Álava and La Rioja, Ferdinand demanded the cession only of Bureba.

[2] Over the next decade, he gradually extended his control over more of the western territory of Navarre at the expense of Sancho IV, although this was accomplished peacefully and is only detectable in the documentary record.

He captured the fortresses of San Esteban de Gormaz, Berlanga and Vadorrey, and afterwards proceeded through Santiuste, Huermeces and Santamara as far as the Roman road that lay between Toledo and Zaragoza.

[19] The success of the campaign was made possible by the preoccupation of the Zaragozan emir, Ahmad al-Muqtadir, with attacking the neighboring taifa of Tortosa and defending his northeastern frontier from Ramiro I of Aragon and Raymond Berengar I of Barcelona.

[18] Having secured the Duero, Ferdinand began to bring the valley of the Mondego under his control, first taking Viseu in its middle stretch on 25 July 1058 and then moving down towards the sea.

[25] After becoming ill during the Siege of Valencia and the Battle of Paterna, Ferdinand died on 24 December 1065, in León,[1] with many manifestations of ardent piety, having laid aside his crown and royal mantle, dressed in the robe of a monk and lying on a bier covered with ashes, which was placed before the altar of the Basilica of San Isidoro.

[26] By his will, Ferdinand divided his kingdom among his three sons: the eldest, Sancho, received Castile; the second, Alfonso, León; and from the latter the region of Galicia was carved off to create a separate state for García.

The Chronicon complutense, probably written shortly after Ferdinand's death, extols him as the "exceedingly strong emperor" (imperator fortissimus) when mentioning the siege of Coimbra.

[30] In the fourteenth century a legend appeared in various chronicles according to which the pope, the Holy Roman emperor, and the king of France demanded a tribute from Ferdinand.

[32] In the sixteenth century this account re-appeared, extended and elaborated, by Juan de Mariana, who wrote that in 1055, at a synod in Florence, the Emperor Henry III urged Victor II to prohibit under severe penalties the use of the imperial title by Ferdinand of León.

[34] German historian E. E. Stengel believed the version found in Mariana on the grounds that the latter probably used the now lost acts of the Council of Florence.

[35] Juan Beneyto Pérez was willing to accept it as based on tradition and Ernst Steindorff, the nineteenth-century student of the reign of Henry III, as being authentically transmitted via the romancero.