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.

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

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