Treaty of Windsor (1386)
| Treaty of peace, friendship and confederation between John I of Portugal and Richard II, King of England | |
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Treaty of Windsor in the Portuguese National Archives | |
| Drafted | 9 May 1386 |
| Signed | 24 February 1387 |
| Location | Westminster |
| Negotiators |
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| Signatories | |
| Ratifiers | |
| Depositary | Portuguese National Archives |
| Language | Latin |
The Treaty of Windsor is a diplomatic alliance signed between the Kingdom of Portugal and the Kingdom of England on 24 February 1387 in Windsor, Berkshire, 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. 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. The Treaty of Windsor established a pact of mutual support.
Copies are preserved at the Torre do Tombo National Archive in Lisbon and The National Archives in London.
The historian Matthew Winslett says, "This treaty has been the cornerstone of both nations' relations with each other ever since." Still in effect today, it is the longest-lasting diplomatic treaty in recorded history.