The siege of Algeciras was the first of many sieges of the city by Christian forces in the lengthy period of the Spanish Reconquista. The siege, ordered by King Alfonso X of Castile also known as "el Sabio", was a fruitless military campaign initiated by the Kingdom of Castile with the objective of removing the Benimerins from Algeciras. The siege on Algeciras, then known to the Muslims as Al-Jazira Al-Khadra, was strategically important because Algeciras had been at the time the main fortress and landing place for African reinforcement troops in the Iberian Peninsula. Castile, which had a powerful armada of ships anchored in the Bay of Gibraltar to blockade such reinforcement, had a few days previously to the siege, seen that fleet obliterated by the Muslim admiral, Abu Yusuf Yaqub at the Naval Battle of Algeciras.

Algeciras is a municipality of Spain belonging to the province of Cádiz, Andalusia. Located in the southern end of the Iberian Peninsula, near the Strait of Gibraltar, it is the largest city on the Bay of Gibraltar.