Abū ʿAlī Manṣūr (13 August 985 – 13 February 1021), better known by his regnal name al-Ḥākim bi-Amr Allāh (Arabic: الحاكم بأمر الله, lit. 'The Ruler by the Order of God'), was the sixth Fatimid caliph and 16th Ismaili imam (996–1021). Al-Hakim is an important figure in a number of Shia Ismaili sects, such as the world's 15 million Nizaris and 1–2 million Musta'lis, in addition to the 2 million Druze of the Levant.

Histories of al-Hakim can prove controversial, as diverse views of his life and legacy exist. Historian Paul Walker writes: "Ultimately, both views of him, the mad and despotic tyrant (like Germanic and Roman despots) irrationally given to killing those around him on a whim, and the ideal supreme ruler, divinely ordained and chosen, whose every action was just and righteous, were to persist, the one among his enemies and those who rebelled against him, and the other in the hearts of true believers, who, while perhaps perplexed by events, nonetheless remained avidly loyal to him to the end." He was known by his critics as the "mad Caliph" or the "Nero of Islam".

His persecution of Christians was a significant precursor to the Crusades, as he not only forbade pilgrimage to the Holy Land but also demolished the Basilica of the Holy Sepulcher in Jerusalem in 1009. However, it is worth noting that the Byzantines reconstructed the Basilica before the Crusades, which means that Al-Hakim's actions cannot be seen as a direct cause of the Crusades.