Alfonso VIII (11 November 1155, Soria – 5 October 1214), called the Noble or el de las Navas, was the King of Castile from 1158 to his death and King of Toledo. He is most remembered for his part in the Reconquista and the downfall of the Almohad Caliphate. After having suffered a great defeat with his own army at Alarcos against the Almohads, he led the coalition of Christian princes and foreign crusaders who broke the power of the Almohads in the Battle of the Navas de Tolosa in 1212, an event which marked the arrival of an irreversible tide of Christian supremacy on the Iberian peninsula. His reign saw the domination of Castile over León and, by his alliance with Aragon, he drew those two spheres of Christian Iberia into close connection.

Alfonso was born to Sancho III of Castile and Blanche, daughter of García Ramírez of Navarre, in Soria on 11 November 1155. He was named after his grandfather Alfonso VII of Castile. His early life resembled that of other medieval kings. His father died in 1158 when his mother was also dead. Though proclaimed king when only three years of age, he was regarded as merely nominal by the unruly nobles to whom a minority was convenient. Immediately, Castile was plunged into conflicts between the various noble houses vying for ascendancy in the inevitable regency. The devotion of a squire of his household, who carried him on the pommel of his saddle to the stronghold of San Esteban de Gormaz, saved him from falling into the hands of the contending factions. The noble houses of Lara and Castro both claimed the regency, as did the boy's uncle, Ferdinand II of León. In 1159 the young Alfonso was put briefly in the custody of García Garcés de Aza, who was not wealthy enough to support him. In March 1160 the Castro and Lara met at the Battle of Lobregal and the Castro were victorious, but the guardianship of Alfonso and the regency fell to Manrique Pérez de Lara.

Alfonso was put in the custody of the loyal village Ávila. At barely fifteen, he came forth to do a man's work by restoring his kingdom to order. It was only by a surprise that he recovered his capital Toledo from the hands of the Laras.

In 1174, he ceded Uclés to the Order of Santiago and afterwards this became the order's principal seat. From Uclés, he began a campaign which culminated in the reconquest of Cuenca in 1177. The city surrendered on 21 September, the feast of Saint Matthew, ever afterwards celebrated by the citizens of the town.

Alfonso took the initiative to ally all the major Christian kingdoms of the peninsula — Navarre, León, Portugal, and Aragon — against the Almohads. By the Treaty of Cazola of 1179, the zones of expansion of each kingdom were defined. After founding Plasencia (Cáceres) in 1186, he embarked on a major initiative to unite the Castilian nobility around the Reconquista. In that year, he recuperated part of La Rioja from the Kingdom of Navarre.

In 1195, after the treaty with the Almohads was broken, he came to the defence of Alarcos on the river Guadiana, then the principal Castilian town in the region. At the subsequent Battle of Alarcos, he was roundly defeated by the caliph Abu Yaqub Yusuf al-Mansur. The reoccupation of the surrounding territory by the Almohads was quickly commenced with Calatrava falling first. For the next seventeen years, the frontier between Moor and Castilian was fixed in the hill country just outside Toledo.

Finally, in 1212, through the mediation of Pope Innocent III, a crusade was called against the Almohads. Castilians under Alfonso, Aragonese and Catalans under Peter II, Navarrese under Sancho VII, and Franks under the archbishop Arnold of Narbonne all flocked to the effort. The military orders also lent their support. Calatrava first, then Alarcos, and finally Benavente were captured before a final battle was fought at Las Navas de Tolosa near Santa Elena on 16 July. The caliph Muhammad an-Nasir was routed and Almohad power broken.

Alfonso was the founder of the first Spanish university, a studium generale at Palencia, which, however, did not survive him. His court also served as an important instrument for Spanish cultural achievement. His marriage (Burgos, before 17 September 1177) with Eleanor (Leonora), daughter of Henry II of England and Eleanor of Aquitaine, brought him under the influence of the greatest governing intellect of his time. Troubadours and sages were always present, largely due to the influence of Eleanor.

Alfonso died at Gutierre-Muñoz and was succeeded by his eldest surviving son, Henry I, named after his maternal grandfather.

Alfonso was the subject for Lion Feuchtwanger's novel Die Jüdin von Toledo (The Jewess of Toledo), in which is narrated an affair with a Jewish subject in medieval Toledo in a time when Spain was known to be the land of tolerance and learning for Jews, Christians, and Muslims. The titular Jewish woman of the novel is based on Alfonso's historical paramour, Rahel la Fermosa.