The Battle of Nájera, also known as the Battle of Navarrete, was fought on 3 April 1367 near Nájera, in the province of La Rioja, Castile. It was an episode of the first Castilian Civil War which confronted King Peter of Castile with his half-brother Count Henry of Trastámara who aspired to the throne; the war involved Castile in the Hundred Years' War. Castilian naval power, far superior to that of France or England, encouraged the two polities to take sides in the civil war, to gain control over the Castilian fleet.

King Peter of Castile was supported by England, Aquitaine, Majorca, Navarra and the best European mercenaries hired by the Black Prince. His rival, Count Henry, was aided by a majority of the nobility and the Christian military organizations in Castile. While neither the Kingdom of France nor the Crown of Aragon gave him official assistance, he had on his side many Aragonese Soldiers and the French free companies loyal to his lieutenant the Breton knight and French commander Bertrand du Guesclin. Although the battle ended with a resounding defeat for Henry, it had disastrous consequences for King Peter and the Prince of Wales and England.