The Treaty of Alcáçovas is signed by the Catholic Monarchs of Castile and Aragon on one side and Afonso V and his son, Prince John of Portugal.

The Treaty of Alcáçovas was signed on 4 September 1479 between the Catholic Monarchs of Castile and Aragon on one side and Afonso V and his son, Prince John of Portugal, on the other side. It put an end to the War of the Castilian Succession, which ended with a victory of the Castilians on land and a Portuguese victory on the sea. The four peace treaties signed at Alcáçovas reflected that outcome: Isabella was recognized as Queen of Castile while Portugal reached hegemony in the Atlantic Ocean.

The Catholic Monarchs were Queen Isabella I of Castile and King Ferdinand II of Aragon, whose marriage and joint rule marked the de facto unification of Spain. They were both from the House of Trastámara and were second cousins, as they were both descended from John I of Castile. To remove the obstacle that this consanguinity would otherwise have posed to their marriage under canon law, they were given a papal dispensation by Sixtus IV. They married on October 19, 1469, in the city of Valladolid; Isabella was 18 years old and Ferdinand a year younger. Most scholars generally accept that the unification of Spain can essentially be traced back to the marriage of Ferdinand and Isabella. Their reign was called by W.H. Prescott "the most glorious epoch in the annals of Spain."

The Crown of Castile was a medieval polity in the Iberian Peninsula that formed in 1230 as a result of the third and definitive union of the crowns and, some decades later, the parliaments of the kingdoms of Castile and León upon the accession of the then Castilian king, Ferdinand III, to the vacant Leonese throne. It continued to exist as a separate entity after the personal union in 1469 of the crowns of Castile and Aragon with the marriage of the Catholic Monarchs up to the promulgation of the Nueva Planta decrees by Philip V in 1716.