Alfonso X (also known as the Wise, Spanish: el Sabio; 23 November 1221 – 4 April 1284) was King of Castile, León and Galicia from 1 June 1252 until his death in 1284. During the election of 1257, a dissident faction chose him to be king of Germany on 1 April. He renounced his claim to Germany in 1275, and in creating an alliance with the Kingdom of England in 1254, his claim on the Duchy of Gascony as well.

Alfonso's scientific interests—he is sometimes nicknamed the Astrologer (el Astrólogo)—led him to sponsor the creation of the Alfonsine tables, and the Alphonsus crater on the Moon is named after him. He also sponsored the work of historians, who for the first time placed Spain in the context of world history. As a lawmaker he introduced the first vernacular law code in Castile, the Siete Partidas. He created the Mesta, an association of sheep farmers in the central plain, but debased the coinage to finance his claim to the German crown. He fought a successful war with Portugal, but a less successful one with Granada. The end of his reign was marred by a civil war with his eldest surviving son, the future Sancho IV, which continued after his death.