constable of Castile, ruler of Castile during much of the reign of the weak John II.

Luna was the illegitimate son of a noble of Aragonese descent and the only distinguished statesman during a dismal period in Castilian history. He was a skilled politician, a farsighted legislator, a competent soldier, and a minor poet and wit. His weakness was that, born without inherited wealth, he used his position to accumulate estates and money. Luna secured the favour of John II at a very early age, but his period of real power began when he rescued the king from the tutelage of rebel nobles (1420) and was appointed constable (1423).

For many years his main efforts were concerned with saving the crown from armed factions of dissident magnates who sought to control it. These twice succeeded in ousting Luna (1427 and 1438), but they proved incapable of governing effectively, and Luna was summoned back to power. The chief rebel leaders were the sons of Ferdinand I of Aragon, who were Castilian magnates in their own right. When they were routed at Olmedo (1445), where Luna as constable led the loyalist vanguard, the rebellions were at last at an end.

However, in 1447 John II married Isabella of Portugal, who determined to destroy Luna's power over her husband. In 1453, Isabella, supported by their son, the future Henry IV, persuaded the king to arrest Luna and have him publicly executed at Valladolid—an event which seems to have led to the king's death, of remorse, a year later.