Genghis Khan (born Temüjin; c. 1162 – August 1227), also known as Chinggis Khan, was the founder and first khan of the Mongol Empire. After spending most of his life uniting the Mongol tribes, he launched a series of military campaigns, conquering large parts of China and Central Asia.

Born between 1155 and 1167 and given the name Temüjin, he was the eldest child of Yesugei, a Mongol chieftain of the Borjigin clan, and his wife Hö'elün. When Temüjin was eight, his father died and his family was abandoned by its tribe. Reduced to near-poverty, Temüjin killed his older half-brother to secure his familial position. His charismatic personality helped to attract his first followers and to form alliances with two prominent steppe leaders named Jamukha and Toghrul; they worked together to retrieve Temüjin's newlywed wife Börte, who had been kidnapped by raiders. As his reputation grew, his relationship with Jamukha deteriorated into open warfare. Temüjin was badly defeated in c. 1187, and may have spent the following years as a subject of the Jin dynasty; upon reemerging in 1196, he swiftly began gaining power. Toghrul came to view Temüjin as a threat and launched a surprise attack on him in 1203. Temüjin regrouped and overpowered Toghrul; after defeating the Naiman tribe and executing Jamukha, he was left as the sole ruler on the Mongolian steppe.

Temüjin formally adopted the title "Genghis Khan", the meaning of which is uncertain, at an assembly in 1206. Carrying out reforms designed to ensure long-term stability, he transformed the Mongols' tribal structure into an integrated meritocracy dedicated to the service of the ruling family. After thwarting a coup attempt from a powerful shaman, Genghis began to consolidate his power. In 1209, he led a large-scale raid into the neighbouring Western Xia, who agreed to Mongol terms the following year. He then launched a campaign against the Jin dynasty, which lasted for four years and ended in 1215 with the capture of the Jin capital Zhongdu. His general Jebe annexed the Central Asian state of Qara Khitai in 1218. Genghis was provoked to invade the Khwarazmian Empire the following year by the execution of his envoys; the campaign toppled the Khwarazmian state and devastated the regions of Transoxiana and Khorasan, while Jebe and his colleague Subutai led an expedition that reached Georgia and Kievan Rus'. In 1227, Genghis died while subduing the rebellious Western Xia; following a two-year interregnum, his third son and heir Ögedei acceded to the throne in 1229.

Genghis Khan remains a controversial figure. He was generous and intensely loyal to his followers, but ruthless towards his enemies. He welcomed advice from diverse sources in his quest for world domination, for which he believed the shamanic supreme deity Tengri had destined him. The Mongol army under Genghis killed millions of people, yet his conquests also facilitated unprecedented commercial and cultural exchange over a vast geographical area. He is remembered as a backwards, savage tyrant in Russia and the Arab world, while recent Western scholarship has begun to reassess its previous view of him as a barbarian warlord. He was posthumously deified in Mongolia; modern Mongolians recognise him as the founding father of their nation.

There is no universal romanisation system used for Mongolian; as a result, modern spellings of Mongolian names vary greatly and may result in considerably different pronunciations from the original. The honorific most commonly rendered as "Genghis" ultimately derives from the Mongolian ᠴᠢᠩᠭᠢᠰ, which may be romanised as Činggis. This was adapted into Chinese as 成吉思 Chéngjísī, and into Persian as چنگیز Čəngīz. As Arabic lacks a sound similar to [tʃ], represented in the Mongolian and Persian romanisations by ⟨č⟩, writers transcribed the name as J̌ingiz, while Syriac authors used Šīngīz.

In addition to "Genghis", introduced into English during the 18th century based on a misreading of Persian sources, modern English spellings include "Chinggis", "Chingis", "Jinghis", and "Jengiz". His birth name "Temüjin" (ᠲᠡᠮᠦᠵᠢᠨ; 鐵木真 Tiěmùzhēn) is sometimes also spelled "Temuchin" in English.

When Genghis's grandson Kublai Khan established the Yuan dynasty in 1271, he bestowed the temple name Taizu (太祖, meaning 'Supreme Progenitor') and the posthumous name Shengwu Huangdi (聖武皇帝, meaning 'Holy-Martial Emperor') upon his grandfather. Kublai's great-grandson Külüg Khan later expanded this title into Fatian Qiyun Shengwu Huangdi (法天啟運聖武皇帝, meaning 'Interpreter of the Heavenly Law, Initiator of the Good Fortune, Holy-Martial Emperor').

As the sources are written in more than a dozen languages from across Eurasia, modern historians have found it difficult to compile information on the life of Genghis Khan. All accounts of his adolescence and rise to power derive from two Mongolian-language sources—the Secret History of the Mongols, and the Altan Debter (Golden Book). The latter, now lost, served as inspiration for two Chinese chronicles—the 14th-century History of Yuan and the Shengwu qinzheng lu (Campaigns of Genghis Khan). The History of Yuan, while poorly edited, provides a large amount of detail on individual campaigns and people; the Shengwu is more disciplined in its chronology, but does not criticise Genghis and occasionally contains errors.