Alfonso III (c. 848–20 December 910), called the Great, was the king of León, Galicia and Asturias from 866 until his death. He was the son and successor of Ordoño I.

Little is known about Alfonso except the bare facts of his reign and of his comparative success in consolidating the kingdom henceforth known as "of Galicia" or "of Oviedo", during the weakness of the Umayyad princes of Cordoba. He fought against and gained numerous victories over the Muslims of al-Andalus, nonetheless his kingdom was always inferior to that of the Cordobans, and he was thus forced to pay them tribute.

He defeated a Basque rebellion in 867 and, much later, a Galician one as well. He conquered Oporto and Coimbra in 868 and 878 respectively. In about 869, he formed an alliance with the Kingdom of Pamplona, and solidified this link by marrying Jimena, who is thought to have been daughter of king García Íñiguez, or less likely, a member of the Jiménez dynasty, and also married his sister Leodegundia to a prince of Pamplona.

He ordered the creation of three chronicles in which was presented the theory that the kingdom of Asturias was the rightful successor of the old Visigothic kingdom. He was also a patron of the arts, like his grandfather before him. He built the church of Santo Adriano de Tuñón.

Alfonso died in Zamora, probably in 910. Upon his death his kingdom was divided among his three sons by Jimena. The eldest son, García, became king of León but died shortly after in 914 without an heir. The second son, Ordoño, reigned in Galicia from 910 and León after García's death. The youngest son, Fruela, received Asturias with Oviedo as his capital. When Ordoño died his children were too young to ascend and the territory of Alfonso was once again united under Fruela, but the latter did not enjoy his joint monarchy for long as he died the next year. Ordoño's eldest son, Alfonso, succeeded him.