Alfonso VI (before June 1040 – July 1, 1109), nicknamed the Brave, was king of León from 1065 to 1109 and king of Castile since 1072 after his brother's death. As he was the first Alfonso to be King of Castile he is sometimes referred to as Alfonso I of Castile. In 1077, he proclaimed himself "emperor of all Spain". Much romance has gathered round his name.

As the second and favorite son of King Ferdinand I of Castile and Princess Sancha of León, Alfonso was alloted León, while Castile was given to the eldest son Sancho, and Galicia to the youngest brother Garcia. Sancho was assasinated in 1072, and Garcia was dethroned and imprisoned for life the following year.

In the cantar de gesta The Lay of the Cid, he plays the part attributed by medieval poets to the greatest kings, and to Charlemagne himself. He is alternately the oppressor and the victim of heroic and self-willed nobles — the idealized types of the patrons for whom the jongleurs and troubadours sang. He is the hero of a cantar de gesta which, like all but a very few of the early Spanish songs, like the cantar of Bernardo del Carpio and the Infantes of Lara, exists now only in the fragments incorporated in the chronicle of Alfonso the Wise or in ballad form.

His flight from the monastery of Sahagun, where his brother Sancho endeavoured to imprison him, his chivalrous friendship for his host Almamun of Toledo, caballero aunque moro, "a knight although a Moor", the passionate loyalty of his vassal Peranzules, and his brotherly love for his sister Urraca of Zamora, may owe something to the poet who took him as a hero.

They are the answer to the poet of the nobles who represented the king as having submitted to taking a degrading oath at the hands of Rodrigo Díaz de Vivar (El Cid) to deny intervention in his brother's death, in the church of Santa Gadea at Burgos, and as having then persecuted the brave man who defied him.

When every allowance is made, Alfonso VI stands out as a strong man fighting as a king whose interest was law and order, and who was the leader of the nation in the reconquest. He impressed himself on the Arabs as a very fierce and astute enemy, but as a keeper of his word. A story of Muslim origin, which is probably no more historical than the oath of Santa Gadea, tells of how he allowed himself to be tricked by Ibn Ammar, the favourite of Al Mutamid, the king of Seville. They played chess for an extremely beautiful table and set of men, belonging to Ibn Ammar. Table and men were to go to the king if he won. If Ibn Ammar gained he was to name the stake. The latter did win and demanded that the Christian king should spare Seville. Alfonso kept his word.

Whatever truth may lie behind the romantic tales of Christian and Muslim, we know that Alfonso represented in a remarkable way the two great influences then shaping the character and civilization of Spain.

Alfonso married at least five times, had two mistresses, and one fiancée. His first wife was Agnes, daughter of William VII of Aquitaine. They married in 1069 and they had no children and were divorced due to consanguinity. The second wife was Constance of Burgundy; their daughter was Urraca of Castile. They married in 1081. Prior to this he was betrothed to Agatha, one of the daughters of William I of England. In 1093, he married Bertha of Burgundy, daughter of William I, palantine count of Burgundy. Then a later wife, Beatrice of unknown origin. By his mistress Jimena Muñoz, daughter of the Count of Asturias, he had an illegitimate daughter, Teresa of Leon and by another mistress, Isabel de Denia, he had another illiegitimate daughter, Elvira of Castile.

At the instigation, it is said, of his wife Constance, he brought the Cistercian Order into Spain, established them in Sahagun, chose a French Cistercian, Bernard, as the first archbishop of Toledo after the reconquest on May 25, 1085. He married his daughters, Urraca of Castile, the legitimate and Teresa of Leon, the illegitimate, to French princes, and in every way forwarded the spread of French influence — then the greatest civilizing force in Europe. He also drew Spain nearer to the Papacy, and it was his decision which established the Roman ritual in place of the old missal of Saint Isidore — the Mozarabic rite.

On the other hand he was very open to Arabic influence. He protected the Muslims among his subjects and struck coins with inscriptions in Arabic letters. After the death of Constance he perhaps married and he certainly lived with Zaida, said to have been a daughter-in-law of Al Mutamid, Muslim king of Seville. Alfonso's wife Isabel, who bore him the only son, Sancho, among his many children, may have been this Zaida, who became a Christian under the name of Maria or Isabel. Isabel also bore him two daughters, Elvira Alfonso (who married Roger II of Sicily) and Sancha (wife of Rodrigo Gonzalez de Lara).

Sancho, Alfonso's designated successor, was slain at the battle of Ucles in 1108.