Abdülhamid II or Abdul Hamid II (Ottoman Turkish: عبد الحميد ثانی, romanized: Abd ul-Hamid-i s̱ānī; Turkish: II. Abdülhamid; 21 September 1842 – 10 February 1918) was the 34th sultan of the Ottoman Empire, from 1876 to 1909, and the last sultan to exert effective control over the fracturing state. He oversaw a period of decline with rebellions (particularly in the Balkans), and presided over an unsuccessful war with the Russian Empire (1877–78), the loss of Egypt, Cyprus, Bulgaria, Serbia, Montenegro, Tunisia, and Thessaly from Ottoman control (1877–1882), followed by a successful war against Greece in 1897, though Ottoman gains were tempered by subsequent Western European intervention.

Elevated to power in the wake of Young Ottoman coups, he promulgated the Ottoman Empire's first constitution, a sign of the progressive thinking that marked his early rule. But his enthronement came in the context of the Great Eastern Crisis, which began with the Empire's default on its loans, uprisings by Christian Balkan minorities, and a war with the Russian Empire. At the end of the crisis, Ottoman rule in the Balkans and its international prestige were severely diminished, and the Empire lost its economic sovereignty as its finances came under the control of the Great Powers through the Ottoman Public Debt Administration.

In 1878, Abdul Hamid consolidated his rule by suspending both the constitution and the parliament, purging the Young Ottomans [tr], and curtailing the power of the Sublime Porte. He ruled as an autocrat for three decades. Ideologically an Islamist, the sultan asserted his title of Caliph to Muslims around the world. His paranoia about being overthrown, like his uncle and half-brother, led to the creation of secret police organizations, such as the Yıldız Intelligence Agency and the Umur-u Hafiye, and a censorship regime. The Ottoman Empire's modernization and centralization continued during his reign, including reform of the bureaucracy, extension of the Rumelia Railway and the Anatolia Railway, and construction of the Baghdad Railway and the Hejaz Railway with German assistance. Systems for population registration, sedentarization of tribal groups, and control over the press were part of a unique imperialist system in fringe provinces known as borrowed colonialism. The farthest-reaching reforms were in education through the establishment of many professional schools and a network of primary, secondary, and military schools throughout the Empire.

Ironically, the same educational institutions that the Sultan sponsored proved to be his downfall. Large sections of the Ottoman intelligentsia were discontent with his repressive policies, which coalesced into the Young Turks movement. Ethnic minorities started organizing their own national liberation movements, resulting in insurgencies in Macedonia and Eastern Anatolia. Armenians especially suffered from massacres and pogroms at the hands of the Hamidiye regiments. Of the many assassination attempts during Abdul Hamid's reign, one of the most famous is the Armenian Revolutionary Federation's Yıldız assassination attempt of 1905. In 1908, the Committee of Union and Progress forced him to recall parliament and reinstate the constitution in the Young Turk Revolution. Abdul Hamid II attempted to reassert his absolutism a year later, resulting in his deposition by pro-constitutionalist forces in the 31 March incident, though the role he played in these events is disputed.

Abdul Hamid has been long vilified as a reactionary "Red Sultan" for his tyrannical leadership and condoning of atrocities. It was initial consensus that his personal rule created an era of stagnation which held the Ottoman Empire back from the otherwise dynamic Belle Époque. Recent assessments have highlighted his promotion of education and public works projects, his reign a culmination and advancement of the Tanzimat reforms. Since the AKP's rise to power, scholars have attributed a resurgence in his personality cult an attempt to check Mustafa Kemal Atatürk's established image as the founder of modern Turkey.

Hamid Efendi was born on 21 September 1842 either in Çırağan Palace, Ortaköy, or at Topkapı Palace, both in Constantinople. He was the son of Sultan Abdulmejid I and Tirimüjgan Kadın (Circassia, 20 August 1819 – Constantinople, Feriye Palace, 2 November 1853), originally named Virjinia. Following his mother's death, he became the adoptive son of his father's legal wife, Perestu Kadın. Perestu was also the adoptive mother of Abdul Hamid's half-sister Cemile Sultan, whose mother Düzdidil Kadın had died in 1845, leaving her motherless at the age of two. The two were brought up in the same household, where they spent their childhood together.

Unlike many other Ottoman sultans, Abdul Hamid II visited distant countries. In the summer of 1867, nine years before he ascended the throne, he accompanied his uncle Sultan Abdul Aziz on a visit to Paris (30 June – 10 July 1867), London (12–23 July 1867), Vienna (28–30 July 1867), and capitals or cities of a number of other European countries.

Abdul Hamid ascended the throne after his brother Murad was deposed on 31 August 1876. At his accession, some commentators were impressed that he rode practically unattended to the Eyüp Sultan Mosque, where he was presented with the Sword of Osman. Most people expected Abdul Hamid II to support liberal movements, but he acceded to the throne at a critical time. Economic and political turmoil, local wars in the Balkans, and the Russo-Turkish War threatened the Empire's very existence.