The Battle of Aljubarrota (Portuguese pronunciation: [alut]; see Aljubarrota) was fought between the Kingdom of Portugal and the Crown of Castile on 14 August 1385. Forces commanded by King John I of Portugal and his general Nuno lvares Pereira, with the support of English allies, opposed the army of King John I of Castile with its Aragonese, Italian and French allies at So Jorge, between the towns of Leiria and Alcobaa, in central Portugal. The result was a decisive victory for the Portuguese, ruling out Castilian ambitions to the Portuguese throne, ending the 138385 Crisis and assuring John as King of Portugal.

Portuguese independence was confirmed and a new dynasty, the House of Aviz, was established. Scattered border confrontations with Castilian troops would persist until the death of John I of Castile in 1390, but these posed no real threat to the new dynasty. To celebrate his victory and acknowledge divine help, John I of Portugal ordered the construction of the monastery of Santa Maria da Vitria na Batalha and the founding of the town of Batalha (pronounced [bta] (listen); Portuguese for '"battle"'), close to the site where the battle was fought. The king, his wife Philippa of Lancaster, and several of his sons are buried in this monastery, which is now a UNESCO World Heritage Site.

The 1383–1385 Portuguese interregnum was a civil war in Portuguese history during which no crowned king of Portugal reigned. The interregnum began when King Ferdinand I died without a male heir and ended when King John I was crowned in 1385 after his victory during the Battle of Aljubarrota.

Portuguese interpret the era as their earliest national resistance movement countering Castilian intervention, and Robert Durand considers it as the "great revealer of national consciousness".The bourgeoisie and the nobility worked together to establish the Aviz dynasty, a branch of the Portuguese House of Burgundy, securely on an independent throne. That contrasted with the lengthy civil wars in France (Hundred Years' War) and England (War of the Roses), which had aristocratic factions fighting powerfully against a centralised monarchy.

It is usually known in Portugal as the 1383-1385 Crisis (Crise de 1383-1385).