The Siege of Lisbon, from July 1 to October 25, 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 and is seen as a pivotal battle of the wider Reconquista.

The Fall of Edessa in 1144 led to a call for a new crusade by Pope Eugene III in 1145 and 1146. In the spring of 1147, the Pope authorized the crusade in the Iberian peninsula. He also authorized Alfonso VII of León to equate his campaigns against the Moors with the rest of the Second Crusade. In May 1147, the first contingents of crusaders left from Dartmouth in England for the Holy Land. Bad weather forced the ships to stop on the Portuguese coast, at the northern city of Porto on June 16, 1147. There they were convinced to meet with King Afonso I of Portugal.

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 July 1. After four months, the Almoravid rulers agreed to surrender on October 24, 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 capital city of the Kingdom of Portugal, in 1255.

The siege began on July 1. The Christians soon captured the surrounding territories and besieged the walls of Lisbon itself, although the Muslim defenders were able to destroy their siege engines.

After four months, the Moorish rulers agreed to surrender (October 21), primarily because of hunger within the city, which was sheltering populations displaced from Santarém as well as "the leading citizens of Sintra, Almada, and Palmela."

After a brief riotous insurrection the Anglo-Norman chronicler attributes to "the men of Cologne and the Flemings", the city was entered by the Christian conquerors, on October 25.