Alfonso XI of Castile (August 13, 1312 – March 26/27, 1350) was the king of Castile and León, the son of Ferdinand IV of Castile and his wife Constance of Portugal.

He is variously known among Castilian kings as the Avenger or the Implacable, and as "he of the Rio Salado." The first two names he earned by the ferocity with which he repressed the disorder of the nobles after a long minority; the third by his victory in battle of Rio Salado over the last formidable African invasion of Spain in 1340.

Alfonso XI never went to the insane lengths of his son Peter I of Castile, but he could be bloody in his methods. He killed for reasons of state without form of trial. He openly neglected his wife, Maria of Portugal (daughter of Afonso IV of Portugal), and had an ostentatious passion for Eleanor of Guzman, who bore him ten children (see her article for the full list). This set Peter an example which he did not fail to better. It may be that his early death, during the great plague of 1350, at the siege of Gibraltar, only averted a desperate struggle with Peter, though it was a misfortune in that it removed a ruler of eminent capacity, who understood his subjects well enough not to go too far.