Sir William II de Longespée (ca. 1207 - February 8, 1250) was the son of William de Longespee, 3rd Earl of Salisbury, an English noble. His death became of significant importance to the English psyche, having died as a martyr due to the purported mistakes, and arrogance, of the French at the Battle of Mansurah, near Al-Mansurah in Egypt.

Longespee made a pilgrimage to the Holy Land in 1240, and again in 1247. The second time, he proceeded to Rome and made a plee to the Pope for support:

• "Sir, you see that I am signed with the cross and am on my journey with the King of France to fight in this pilgrimage. My name is great and of note, viz., William Longespee, but my estate is slender, for the king of England, my kinsman and liege lord, hath bereft me of the title of earl and of that estate, but this he did judiciously, and not in displeasure, and by the impulse of his will; therefore I do not blame him for it. Howbeit, I am necessitated to have recourse to your holiness for favour, desiring your assistance in this distress. We see here (quoth he) that Earl Richard (of Cornwall) who, though he is not signed with the cross, yet, through the especial grace of your holiness, he hath got very much money from those who are signed, and therefore, I, who am signed and in want, do intreat the like favour."

Having succeeded in gaining the favour of the Pope, Longespee raised a company of 200 English horse to join with Louis IX on his crusade.

During the Seventh Crusade, Longespee commanded the English forces. He became widely known for his feats of chivalry and his subsequent martyrdom. The circumstances of his death served to fuel growing English animosity toward the French; it is reported that the French Count d'Artois lured Longespee into attacking the Mameluks before the forces of King Louis IX arrived in support. Robert d'Artois, William II Longespee and his men, along with 280 Templars, were killed at this time.

It is said that his mother, Abbess Ela Longespee, had a vision of the martyr being received into heaven by angels just one day prior to his death.

In 1252, the Sultan delivered Longespee's remains to a messenger who conveyed them to Acre (Akko) for burial at the church of St. Cross. However, his effigy is found amongst family members at Salisbury Cathedral, in England.

By his wife Idoine de Camville, Sir William had two sons and two daughters:

• Ida de Longespee, who first married Ralph de Somery, and then William Beauchamp
• Ela, who married James of Aldithley
• William III de Longespee
• Richard Longespee