Alfonso X (23 November 1221 – 4 April 1284) was a Castilian monarch who ruled as the King of Castile, León and Galicia from 1252 until his death. He also was elected King of the Germans in 1257.

He established Castilian as a language of higher learning and earned his nicknames "the Wise" or "the Learned" (Spanish: 'el Sabio', Galician: 'O Sabio') and "the Astrologer" (Spanish: 'el Astrólogo', Galician: 'O Astrólogo') through his own prolific writings, including Galician poetry.

Born in Toledo, Spain, Alfonso was the eldest son of Ferdinand III of Castile and Elisabeth of Hohenstaufen, known in Spain as Beatriz de Suabia, through whom he was a cousin of Frederick II, Holy Roman Emperor, to whom Alfonso is often compared. His maternal grandparents were Philip of Swabia and Irene Angelina. Not much is known about his upbringing, but he was most likely raised in Toledo. He began his career as a soldier, under the command of his father, when he was only sixteen years old.

After the election of Theobald I as king of Navarre, his father tried to arrange a marriage for Ferdinand with Theobald's daughter, Blanche of Navarre, but the move was unsuccessful. So, in 1240, he married Maior Guillen de Guzman, but the marriage was later annulled and their issue declared illegitimate. In the same period (1240 – 1250) he conquered several Muslim strongholds in Al-Andalus alongside his father, such as Murcia, Alicante and Cadiz.

In 1249, Alfonso X married Violante of Aragon, the daughter of King James I of Aragon and Yolande of Hungary, although betrothed already in 1246. At the same time he also had a romantic relationship with Sofia Mayor Guillén de Guzmán, who bore to him illegitimate daughter Beatrice.

He succeeded his father as King of Castile and León in 1252. The following year he invaded Portugal, capturing the region of the Algarve. King Afonso III of Portugal had to surrender, but he gained an agreement by which, after he consented to marry Alfonso X's daughter Beatrice of Castile, the land would be returned to their heirs. In 1263 Alfonso X returned Algarve to the King of Portugal.

In 1254 Alfonso X signed a treaty of alliance with the King of England and Duke of Aquitaine, Henry III, supporting him in the war against Louis IX of France. In the same year Alfonso's sister, Eleanor of Castile, married Henry's heir to the throne, Edward: with this act Alfonso renounced forever all claim to the Duchy of Gascony, to which Castile had been a pretender since the marriage of Alfonso VIII of Castile with Eleanor of England and Gascony.

In 1256, at the death of William II of Holland, Alfonso's descent from the Hohenstaufen through his mother, a daughter of the emperor Philip of Swabia, gave him a claim to represent the Swabian line. Alfonso's election as King of the Romans by the imperial prince - electors misled him into complicate schemes that involved excessive expense but never took effect. His rival, Richard of Cornwall, went to Germany and here was also crowned in 1257 at Aachen. Alfonso instead never moved to Germany, and his alliance with the Italian Ghibelline lord Ezzelino IV da Romano deprived him of the initial support of Pope Alexander IV. In the end, after Richard's death, the German princes elected Rudolph I of Habsburg (1272), Alfonso being declared deposed by Pope Gregory X. In 1275 Alfonso tried to meet with his nominal imperial vicar in Italy, William VII of Montferrat (who had succeeded Ezzelino) and his Ghibelline allies in Piedmont and Lombardy to fight against the Guelph Charles I of Anjou; he was however stopped in Provence by the Pope who, after a long negotiation, obtained Alfonso's renunciation of the title of King of the Romans.

To obtain money, he debased the coinage and then endeavoured to prevent a rise in prices by an arbitrary tariff. The little trade of his dominions was ruined, and the burghers and peasants were deeply offended. His nobles, whom he tried to cow by sporadic acts of violence, rebelled against him in 1272. Reconciliation was bought by Alfonso's son Ferdinand in 1273.

In 1273, he created the Mesta, an association of some 3,000 petty and great sheep holders in Castile, in reaction to less wool being exported from the traditional sites in England. This organization later became exceedingly powerful in the country (as wool became Castile's first major exportable commodity), and eventually its privileges were to prove a deadly wound in the Castilian economy. One side effect of the quickly expanding sheep herds was the decimation to the Castilian farmland through which the sheep grazed.

Throughout his reign, Alfonso contended with the nobles, particularly the families of Nuño González de Lara, Diego López de Haro and Esteban Fernández de Castro, all of whom were formidable soldiers and instrumental in maintaining Castile's military strength in frontier territories. According to some scholars, Alfonso lacked the singleness of purpose required by a ruler who would devote himself to organization, and also the combination of firmness with temper needed for dealing with his nobles. Others have argued that his efforts were too singularly focused on the diplomatic and financial arrangements surrounding his bid for the Holy Roman Emperor.

Alfonso's eldest son, Ferdinand, died in 1275 at the battle of Écija against the Moroccan and Granadan invasion armies, leaving two infant sons. Alfonso's second son, Sancho, claimed to be the new heir, in preference to the children of Ferdinand de la Cerda, basing his claim on an old Castilian custom, that of proximity of blood and agnatic seniority. Alfonso preferred to leave the throne to his grandsons, but Sancho had the support of the nobility. A bitter civil war broke out resulting in Alfonso's being forced in 1282 to accept Sancho as his heir instead of his young grandsons; only the cities of Seville, Murcia and Badajoz remained faithful to him. Son and nobles alike supported the Moors when he tried to unite the nation in a crusade; and when he allied himself with Abu Yusuf Yakub, the ruling Marinid Sultan of Morocco, they denounced him as an enemy of the faith. A reaction in his favor was beginning in his later days, but he died defeated and deserted at Seville in 1284, leaving a will, by which he endeavored to exclude Sancho, and a heritage of civil war.

As a ruler, Alfonso showed legislative capacity, and a wish to provide the kingdoms expanded under his father with a code of laws and a consistent judicial system. The Fuero Real was undoubtedly his work. He began medieval Europe's most comprehensive code of law, the Siete Partidas, which, however, thwarted by the nobility of Castile, was only promulgated by his great - grandson. Because of this, and because the Partidas remain fundamental law in the American Southwest, he is one of the 23 lawmakers depicted in the House of Representatives chamber of the United States Capitol.

Alfonso "turned to the vernacular for the kind of intellectual commitments that formerly were inconceivable outside Latin." He was the first king who initiated the use of the Castilian language extensively, although his father, Ferdinand III, had begun to use it for some documents, instead of Latin, as the language used in courts, churches, and in books and official documents.

As an intellectual he gained considerable scientific fame based on his encouragement of astronomy, which included astrology at the time and the Ptolemaic cosmology as known to him through the Arabs. He surrounded himself with mostly Jewish translators who rendered Arabic scientific texts into Castilian at Toledo. His fame extends to the preparation of the Alfonsine tables, based on calculations of al-Zarqali, "Arzachel". Because of this, the lunar crater Alphonsus is named after him. One famous apocryphal quote attributed to him upon hearing an explanation of the extremely complicated mathematics required to demonstrate Ptolemy's theory of astronomy was "If the Lord Almighty had consulted me before embarking on creation thus, I should have recommended something simpler." The validity of this quotation is questioned by some historians. Alfonso also wrote one of the first western chess treatises.

From the beginning of his reign, Alfonso employed Jewish, Christian and Muslim scholars at his court, primarily for the purpose of translating books from Arabic and Hebrew into Latin and Castilian, although he always insisted in supervising personally the translations. This group of scholars, specialized in Hebrew, Arabic and Latin, formed his royal scriptorium, known as the Escuela de Traductores de Toledo (Toledo School of Translators). Their final output promoted Castilian as a learning language both in science and literature, and established the foundations of the new Spanish language. This evolved version of the Castilian language also acquired significant relevance in the royal chancery, where it came to replace Latin, that until then was the language commonly used by the royal diplomacy in Castilla and León.

Most of the first translated texts at the time survive in only one manuscript. The first translation, commissioned by his brother, Fernando de la Cerda — who had extensive experience, both diplomatic and military, among the Muslims of southern Iberia and north Africa — was a Castilian version of the animal fable Kalila wa-Dimna, a book that belongs to the genre of wisdom literature labeled Mirrors for Princes: stories and sayings meant to instruct the monarch in proper and effective governance.

The primary intellectual work of these scholars centered on astronomy and astrology. The early period of Alfonso's reign saw the translation of selected works of magic (Lapidario, Picatrix, Libro de las formas et las ymagenes) all translated by a Jewish scholar named Yehudah ben Moshe (Yhuda Mosca, in the Old Spanish source texts). These were all highly ornate manuscripts (only the Lapidario survives in its entirety) containing what was believed to be secret knowledge on the magical properties of stones and talismans. In addition to these books of astral magic, Alfonso ordered the translation of well known Arabic astrological compendia, including the Libro de las cruzes and Libro conplido en los iudizios de las estrellas. The first of these was, ironically, translated from Latin (it was used among the Visigoths), into Arabic, and then back into Castilian and Latin.

Alfonso also commissioned a compilation of chronicles, the Crónica general, completed in 1264. This work enjoyed renewed popularity starting in the sixteenth century, when there was a revival of interest in history; Florián de Ocampo published a new edition and Lorenzo de Sepúlveda used it as the chief source of his popular romances. Sepúlveda wrote a number of romances having Alfonso X as their hero.
Alfonso also had Libro de ajedrez, dados, y tablas ("Libro de los Juegos" (The Book of Games)) translated into Castilian from Arabic and added illustrations with the goal of perfecting the work. It was completed in 1283.

Alfonso X commissioned or co-authored numerous works of music during his reign. These works included Cantigas d'escarnio e maldicer and the vast compilation Cantigas de Santa Maria ("Songs to the Virgin Mary"), which was written in Galician - Portuguese and figures among the most important of his works. The Cantigas form one of the largest collections of vernacular monophonic songs to survive from the Middle Ages. They consist of 420 poems with musical notation. The poems are for the most part on miracles attributed to the Virgin Mary. One of the miracles Alfonso relates is his own healing in Puerto de Santa María.

Violante was ten years old at the time of her marriage to Alfonso; she produced no children for several years and it was feared that she was barren. Alfonso almost had their marriage annulled, but they went on to have eleven children. Alfonso X also had several illegitimate children. His illegitimate daughter, Beatrice, married King Afonso III of Portugal. An illegitimate son, Martin, was Abbot of Valladolid.