Bermudo III (1017 – 4 September 1037), king of León (1028 – 4 September 1037), son of Alfonso V of León by his first wife Elvira Menéndez, was the last scion of Peter of Cantabria to rule in the Leonese kingdom. He was called Emperor in Galicia in 1025.

Bermudo III was the son of Alfonso V of León by his first wife Elvira Menéndez. He succeeded to the throne of León in 1027. Bermudo married Jimena Sánchez, who was a daughter of Sancho Garcia of Castile.

In 1029, Count García Sánchez of Castile was about to be married to Sancha of León, the elder sister of Bermudo, an arrangement apparently sanctioned by the king of Navarre, when the count was murdered in the city of León by the Velas, a party of Castilian nobles exiled from their own country, who had taken refuge in Leon. Leon and Navarre disputed the succession to the Countship of Castile thus left vacant.

Sancho III of Pamplona was married to Muniadona, daughter of Sancho García of Castile, and sister to the murdered count. Sancho claimed the county of Castile in his wife's name and installed in it their son, Ferdinand, as the new count of Castile. He seized the borderlands between the Cea and the Pisuerga rivers, right above León capital, long a bone of contention between León and Castile. In 1032 Sancho of Navarre forced a marriage between his son, Fernando of Castile, and Sancha of León, and those lands went to Castile as part of her dowry.

In 1034, Sancho wrested the city of León itself from his brother-in-law, Bermudo, who retreated into Galicia. By the time Sancho died in 1035, the meseta north of the Duero was dominated by the Pyrenean pocket kingdom of Navarre.

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