The Treaty of Windsor is a diplomatic alliance signed between Portugal and England on 9 May 1386 in Windsor and sealed by the marriage of King John I of Portugal (House of Aviz) to Philippa of Lancaster, daughter of John of Gaunt, 1st Duke of Lancaster.^[1] With the victory at the Battle of Aljubarrota, assisted by English archers, John I was recognised as the undisputed King of Portugal, putting an end to the interregnum of the 1383–1385 Crisis.^[1] The Treaty of Windsor established a pact of mutual support between the countries.^[1]

This document is preserved at the Portuguese National Archives.^[2]

Historian Matthew Winslett says, "This treaty has been the cornerstone of both nations' relations with each other ever since."^[3]

Enduring to this day, the Treaty of Windsor holds the record as the longest-lasting diplomatic treaty in recorded history.^[4]

• Anglo-Portuguese Treaty of 1373
• British Ultimatum of 1890 – British diplomatic ultimatum to Portugal regarding Portugal's claims in AfricaPages displaying short descriptions of redirect targets
• Timeline of Portuguese history
• Anglo-Portuguese Alliance – 1386 alliance between the UK and Portugal
• Treaty of Tagilde – Anglo-Portuguese treaty signed in 1372