João I, King of Portugal KG (pron. IPA ), in English, John I (the Good or sometimes, the Great or even the One of Good Memory) (Lisbon, April 11 1357 – August 14 1433 in Lisbon) was the 10th king of Portugal and Algarve and the first to use the title Lord of Ceuta. He was the natural son of Pedro I by a noble Galician lady called Teresa Lourenço. In 1364 he was created grand-master of the Order of Aviz. He became king of Portugal and Algarve in 1385, after the 1383–1385 Crisis.



On the death of his lawful brother Fernando in October 1383, without a male heir, strenuous efforts were made to secure the succession for princess Beatrice, his only daughter. As heiress-apparent Beatrice had been married to king John I of Castile, but the popular voice declared against an arrangement by which Portugal would virtually have become united with Castile. The 1383–1385 Crisis followed as a period of political anarchy, when no king ruled the country.

On April 6 1385, the council of the kingdom (cortes in Portuguese) met in Coimbra and declared João, then Master of Aviz, king of Portugal. This was in effect a declaration of war against Castile and its claims to the Portuguese throne. Soon after, the king of Castile invaded Portugal, with the purpose of conquering Lisbon and removing João I from the throne. Juan I was accompanied by French allied cavalry as English troops and generals took the side of João (see Hundred years war). João I then named Nuno Alvares Pereira, his loyal and talented supporter, general and protector of the Kingdom. The invasion was repelled during the Summer after the Battle of Atoleiros, but especially after the decisive battle of Aljubarrota (August 14, 1385), where the Castilian army was virtually annihilated. Juan I of Castile then retreated and the stability of João I's throne was permanently secured.


In 1387, João I married Philippa of Lancaster, daughter of John of Gaunt who had proved to be a worthy ally, consolidating the union of the Anglo-Portuguese Alliance that endures to the present day.

After the death of Juan of Castile in 1390, without leaving issue by Beatrice, João I ruled in peace and pursued the economic development of the country. The only significant military action was the siege and conquest of the city of Ceuta in 1415. By this step he aimed to control navigation of the African coast. But in longer perspective, this was the first step opening the arabian world to medieval Europe, which in fact led to the age of sailing across whole world.

Contemporaneous writers describe him as a man of wit, very keen on concentrating the power on himself, but at the same time with a benevolent and kind personality. His youth education as master of a religious order made him an unusually learned king in the Middle Ages. His love for knowledge and culture was passed to his sons: Duarte, the future king, was a poet and a writer, Pedro, the duke of Coimbra, was one of the most learned princes of his time and Prince Henry the Navigator, the duke of Viseu, started a school of navigation and invested heavily in science and development of nautical topics. In 1430, his only surviving daughter, Isabella, married Philip III, Duke of Burgundy and enjoyed an extremely refined court in his lands; she was the mother of Charles the Bold.