The Hashemite Kingdom of Hejaz (Arabic: المملكة الحجازية الهاشمية, romanized: Al-Mamlakah al-Ḥijāziyyah Al-Hāshimiyyah) was a state in the Hejaz region of Western Asia that included the western portion of the Arabian Peninsula that was ruled by the Hashemite dynasty. It was self-proclaimed as a kingdom in June 1916 during the First World War, to be independent from the Ottoman Empire, on the basis of an alliance with the British Empire to drive the Ottoman Army from the Arabian Peninsula during the Arab Revolt.

The British government had promised Hussein bin Ali, King of Hejaz, a single independent Arab state that would include, in addition to the Hejaz region, modern-day Jordan, Iraq, and most of Syria, with the fate of the Palestine region (today's Israel and Palestine) being mentioned in more ambiguous terms. However, at the end of the First World War, the Treaty of Versailles turned Syria into a French League of Nations mandate and Iraq, Mandate Palestine and Transjordan into British mandates. Hashemite princes were installed as monarchs under the British mandates in Transjordan and Iraq; this became known as the Sharifian solution.

Relations with the British Empire further deteriorated when more European Jews moved to Palestine, as the area was defined under the British rule. Hussein refused to ratify the 1919 Treaty of Versailles, and, in response to a 1921 British proposal to sign a treaty accepting the Mandate system, stated that he could not be expected to "affix his name to a document assigning Palestine to the Zionists and Syria to foreigners". A further British attempt to reach a treaty failed in 1923–24 and negotiations were suspended in March 1924; within six months the British withdrew their support in favour of their central Arabian ally Ibn Saud, who proceeded to conquer Hussein's kingdom.

On 23 September 1932, the Kingdom of Hejaz and Nejd was unified with the other Saudi dominions, creating the unified Kingdom of Saudi Arabia.

In 1908, the Young Turks took over the Ottoman Empire, and in 1909 when a counter-coup failed, the Young Turks "secularized" the government. Hussein bin Ali, Sharif of Mecca, was appointed by the previous Sultan of the Ottoman Empire and did not favor the Young Turks; his opposition to the empire grew over time, culminating in the Arab Revolt.

Hussein bin Ali, the Sharif and Emir of Mecca from 1908, enthroned himself as King of the Hejaz after proclaiming the Great Arab Revolt against the Ottoman Empire, and continued to hold both of the offices of Sharif and King from 1916 to 1924. At the end of his reign he also briefly laid claim to the office of Sharifian Caliph; he was a 37th-generation direct descendant of Muhammad, as he belongs to the Hashemite family. A member of the Dhawu Awn clan (Banu Hashim) from the Qatadid emirs of Mecca, he was perceived to have rebellious inclinations and in 1893 was summoned to Istanbul, where he was kept on the Council of State. In 1908, in the aftermath of the Young Turk Revolution, he was appointed Emir of Mecca by the Ottoman sultan Abdul Hamid II. In 1916, with the promise of British support for Arab independence, he proclaimed the Great Arab Revolt against the Ottoman Empire, accusing the Committee of Union and Progress of violating tenets of Islam and limiting the power of the sultan-caliph. Shortly after the outbreak of the revolt, Hussein declared himself "King of the Arab Countries". However, his pan-Arab aspirations were not accepted by the Allies, who recognized him only as King of the Hejaz.

In the aftermath of World War I, Hussein refused to ratify the Treaty of Versailles, in protest at the Balfour Declaration and the establishment of British and French mandates in Syria, Iraq, and Palestine. He later refused to sign the Anglo-Hashemite Treaty and thus deprived himself of British support when his kingdom was attacked by Ibn Saud. After the Kingdom of Hejaz was invaded by the Al Saud-Wahhabi armies of the Ikhwan, on 23 December 1925 King Hussein bin Ali surrendered to the Saudis, bringing both the Kingdom of Hejaz and the Sharifate of Mecca to an end.

In their capacity as Caliphs, the Sultans of the Ottoman Empire would appoint an official known as the Sharif of Mecca. The role went to a member of the Hashemite family, but the Sultans typically promoted Hashemite intra-familial rivalries in their choice, preventing the building of a solid base of power in the Sharif.