The siege of Alcácer do Sal lasted from 30 July to 18 October 1217. The well fortified city of Alcácer do Sal was a frontier outpost of the Almohad Caliphate facing Portugal. It was besieged by forces from Portugal, León, the military orders and the Fifth Crusade. The latter were led by Count William I of Holland. The expedition was the brainchild of Bishop Soeiro II of Lisbon, whose diocese was threatened by regular raids from Alcácer. King Afonso II of Portugal did not take part in person, but the city was incorporated into his kingdom after its capitulation. The crusaders who took part in the siege, mainly from the Rhineland and the Low Countries, did so without papal authorization and were afterwards ordered to continue on to the Holy Land.

The Almohad Caliphate or Almohad Empire was a North African Berber Muslim empire founded in the 12th century. At its height, it controlled much of the Iberian Peninsula (Al-Andalus) and North Africa.