The Treaty of Windsor is the diplomatic alliance signed between Portugal and England on 9 May 1386 at 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. 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 13831385 Crisis. The Treaty of Windsor established a pact of mutual support between the countries.This document is preserved at the Portuguese National Archives.Historian Matthew Winslett says, "This treaty has been the cornerstone of both nations' relations with each other ever since."

The Anglo-Portuguese Alliance (or Aliança Luso-Inglesa, "Luso-English Alliance") is the oldest alliance based on known history in the world that is still in force by politics. It was signed at the Treaty of Windsor in 1386, between the Kingdom of England (since succeeded by the United Kingdom) and the Kingdom of Portugal (now the Portuguese Republic), though the countries were previously allied via the Anglo-Portuguese Treaty of 1373.

Historically, the Kingdom of Portugal and the Kingdom of England, and later the modern Portuguese Republic and United Kingdom, have never waged war against each other nor have they participated in wars on opposite sides as independent states since the signing of the Treaty of Windsor. While Portugal was subsumed under the Iberian Union, rebellious Portuguese factions and government in exile sought refuge and help in England. England spearheaded the Anglo-Spanish War (1585–1604) on the side of the deposed Portuguese royal house.

The alliance has served both countries throughout their respective military histories, influencing the participation of the United Kingdom in the Peninsular War, the UK's major land contribution to the Napoleonic Wars and the establishment of an Anglo-American base in Portugal. Portugal aided England (and later the UK) in times of need, for example, in the First World War. Today, Portugal and the United Kingdom are both part of NATO, a larger intergovernmental military alliance between several North American and European states that accounts for over 70% of total global military spending.