The siege of Lisbon, from 1 July to 25 October 1147, was the military action that brought the city of Lisbon under definitive Portuguese control and expelled its Moorish overlords. The siege of Lisbon was one of the few Christian victories of the Second Crusade—it was "the only success of the universal operation undertaken by the pilgrim army", i.e., the Second Crusade, according to the near contemporary historian Helmold, though others have questioned whether it was really part of that crusade. It is seen as a pivotal battle of the wider Reconquista.

The crusaders agreed to help the King attack Lisbon, with a solemn agreement that offered to the crusaders the pillage of the city's goods and the ransom money for expected prisoners. The siege began on 1 July. The city of Lisbon at the time of arrival consisted of sixty thousand families, including the refugees who had fled Christian onslaught from neighbouring cities of Santarém and others.

After four months, the Moorish rulers agreed to surrender on 24 October, primarily because of hunger within the city. Most of the crusaders settled in the newly captured city, but some of the crusaders set sail and continued to the Holy Land. Lisbon eventually became the capital city of the Kingdom of Portugal, in 1255.