In fact Lisbon has never been declared or confirmed as the capital in any official document, unlike most other capital cities. It simply became the de facto capital when in 1255 Alfonso III of Portugal moved the court to what had become Portugal’s largest and most important city.
Perhaps to make up for this lack of official status, apart from being the capital of Portugal the city is now also the capital of the District of Lisbon, the capital of the Region of Lisbon and last but not least the capital of the Metropolitan Area of Lisbon.