Sayyid Hussein bin Ali, GCB (1854 – June 4, 1931) (حسین بن علی; Ḥusayn bin ‘Alī), was the Sharif of Mecca, and Emir of Mecca, from 1908 until 1917, when he proclaimed himself King of the Hejaz, which received international recognition. He initiated the Arab Revolt in 1916 against the increasingly nationalistic Ottoman Empire during the course of the First World War. In 1924, when the Ottoman Caliphate was abolished, he further proclaimed himself Caliph of all Muslims. He ruled Hejaz until 1924, when, defeated by Abdul Aziz al Saud, he abdicated the kingdom and other secular titles to his eldest son Ali.

Hussein bin Ali was born in 1853 in Istanbul as the eldest son of Sharif Ali ibn Muhammad and his wife, Salha Bani-Shahar. He was the last of the Hashemite rulers over the Hejaz to be appointed by the Ottoman Sultan. The Hashemites and their followers believe that they descended from the Islamic prophet Muhammad and therefore are highly respected amongst such Muslims.

The Sharif of Mecca (Arabic: شريف مكة‎, Sharīf Makkah) or Hejaz (Arabic: شريف الحجاز‎, Sharīf al-Ḥiǧāz) was the title of the former governors of Hejaz and a traditional steward of the holy cities of Mecca and Medina. The term sharif means "noble" in Arabic and is used to describe the descendants of Muhammad's grandson al-Hassan ibn Ali.

The Sharif was charged with protecting the cities and their environs and ensuring the safety of pilgrims performing the Hajj. The title is sometimes spelled Sheriff or Sherif, with the latter variant used, for example, by T. E. Lawrence in Seven Pillars of Wisdom. The common-law political and legal office of sheriff found in some anglophone countries is unrelated.

The office of the Sharifate of Mecca dates back to the late Abbasid era. Since 1201, the Sharifate was held by a member of the Hawashim clan, not to be confused with the larger clan of Banu Hashim to which all Sharifs claim descent. Descendants of this family continued to hold the position until the Twentieth Century on behalf of various Muslim powers including the Ayyubids and the Mamelukes. In 1517, the Sharif acknowledged the supremacy of the Ottoman Caliph, but maintained a great degree of local autonomy. During the Ottoman era, the Sharifate expanded its authority northwards to include Medina, and southwards to the frontiers of 'Asir, and regularly raided Nejd.