The Treaty of Tagilde was signed between Ferdinand I of Portugal and representatives of John of Gaunt of England, marking the beginning of the Anglo-Portuguese Alliance, which remains in effect today.

The Treaty of Tagilde is a treaty signed on 10 July 1372 in Tagilde, a village in Portugal. It was signed by King Ferdinand I of Portugal and representatives of John of Gaunt, Duke of Lancaster, a claimant to the Crown of Castile and son of the English king, Edward III. This agreement is considered to have constituted the first legal foundation of the Anglo-Portuguese Alliance, which continues to exist. The agreement was followed up by the Anglo-Portuguese Treaty of 1373, known as the Treaty of Peace, Friendship and Alliance, which was signed in St Paul’s Cathedral by King Edward III of England and King Ferdinand and Queen Leonor of Portugal and is also known as the Treaty of London. Subsequently, the Treaty of Windsor (1386) ratified the London treaty. It is the oldest continuous treaty in effect to this day.