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 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 between the countries.

John I, also called John of Aviz, was King of Portugal from 1385 until his death in 1433. He is recognized chiefly for his role in Portugal's victory in a succession war with Castile, preserving his country's independence and establishing the Aviz dynasty on the Portuguese throne. His long reign of 48 years, the most extensive of all Portuguese monarchs, saw the beginning of Portugal's overseas expansion. John's well-remembered reign in his country earned him the epithet of Fond Memory ; he was also referred to as "the Good", sometimes "the Great", and more rarely, especially in Spain, as "the Bastard" (Bastardo).