Bermudo III or Vermudo III (c. 1017 – 4 September 1037) was the king of León from 1028 until his death. He was a son of Alfonso V of León by his first wife Elvira Menéndez, and was the last scion of Peter of Cantabria to rule in the Leonese kingdom. Like several of his predecessors, he sometimes carried the imperial title (as in a document of 1025). He was a child when he succeeded his father. In 1034 he was chased from his throne by King Sancho III of Pamplona and forced to take refuge in Galicia. He returned to power, but was defeated and killed fighting against his brother-in-law, Ferdinand of Castile, in the battle of Tamarón.

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 King Sancho III of Pamplona.

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 Pamplona forced a marriage between his son, Fernando of Castile, and Sancha of León, and those lands went to Castile as part of her dowry.

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