Peter I (Portuguese: Pedro, pronounced [ˈpeðɾu]) (23 February 1187 – 2 June 1258) was the second son of King Sancho I of Portugal and his wife Dulce, infanta of Aragon, and would eventually become Count of Urgell and Lord of the Balearic Islands.

After the death of his father, Peter took the side of his sisters Mafalda, Sancha and Theresa, in their quarrel with his elder brother, now King Afonso II, over inheritance of the castles of Seia, Alenquer and Montemor-o-Novo, Peter got the protection of his sister Theresa, then Queen of León, from whose territory he launched several inconclusive attacks on the Portuguese border province of Trás-os-Montes e Alto Douro, but eventually had to concede defeat and perpetual exile from Portugal.

Peter then left León to become a mercenary in the service of Yusuf II, the Almohad Caliph, commanding a troop of Christian exiles and adventurers in Marrakesh.

However, the Franciscans managed to avoid Peter and his men, and repeated their act, highly sacrilegious in Muslim terms, in the market-place of Marrakesh - leading to their being decapitated by the Caliph personally.

Subsequently, Peter moved to Aragon, his mother's homeland, where he became involved in the schemes and campaigns of his relative,[4] the young and ambitious King James I.