The Epic of King Gesar (Tibetan: གླིང་གེ་སར།, Wylie: gling ge sar), also spelled Kesar (/ˈkɛzər, ˈkɛs-/) or Geser (especially in Mongolian contexts), is a traditional epic originating from Tibet and Central Asia. Folk balladeers continued to transmit the story orally, enriching its plot and embellished its language over time. The narrative reached its "final" form and peak popularity in the early 12th century.

The epic recounts the heroic deeds of the culture hero Gesar, the fearless lord of the legendary kingdom of Ling (Wylie: gling). It is preserved in both poetry and prose, primarily through oral poetic performance, and is widely sung across Central Asia and South Asia. Its classic version originates in central Tibet.

Approximately 100 bards of this epic (Wylie: sgrung, "tale") remain active today in the Gesar belt of China. Tibetan, Mongolian, Buryat, Balti, Ladakhi, and Monguor singers continue to uphold the oral tradition, and the epic has drawn significant scholarly interest as one of the few surviving oral epic traditions still performed as a living art. Versions of the epic among the Yugur and Salar peoples have also been recorded among the Balti of Baltistan, the Burusho people of Hunza and Gilgit, and the Kalmyk and Ladakhi people in Nepal, and various Altai, Turkic, and Tungus tribes. The first printed version was a Mongolian text published in Beijing in 1716.

There are numerous versions of the epic, each with many variants, and some sources regard it as the longest in the world. Although no single definitive text exists, the Chinese compilation of Tibetan versions has so far filled approximately 120 volumes, comprising more than one million verses divided into 29 “chapters.” Western estimates refer to more than 50 distinct editions published to date in China, India, and Tibet.

It has been proposed on the basis of phonetic similarities that the name Gesar reflects the Roman title Caesar, and that the intermediary for the transmission of this imperial title from Rome to Tibet may have been a Turkic language, since kaiser (emperor) entered Turkic through contact with the Byzantine Empire, where Caesar (Καῖσαρ) was an imperial title. The medium for this transmission may have been via Mongolian Kesar. The Mongols were allied with the Byzantines.

Numismatic evidence and some accounts speak of a Bactrian ruler Phrom-kesar, specifically the Kabul Shahi of Gandhara, which was ruled by the Turkic king Fromo Kesaro ("Caesar of Rome"), who was father-in-law of the king of the Kingdom of Khotan around the middle of the 8th century CE. In early Bon sources, From Kesar is always a place name, and never refers, as it does later, to a ruler. In some Tibetan versions of the epic, a king named Phrom Ge-sar or Khrom Ge-sar figures as one of the kings of the four directions – the name is attested in the 10th century and this Phrom/Khrom preserves an Iranian form (*frōm-hrōm) for Rūm/Rome. This eastern Iranian word lies behind the Middle Chinese word for (Eastern) Rome (拂菻, Fólín), namely Byzantium (phrōm-from<*phywət-lyəm>).

A. H. Francke thought the Tibetan name Gesar derived from Sanskrit. S.K. Chatterji, introducing his work, noted that the Ladakh variant of Kesar, Kyesar, in Classical Tibetan Skye-gsar meant 'reborn/newly born', and that Gesar/Kesar in Tibetan, as in Sanskrit signifies the 'anther or pistil of a flower', corresponding to Sanskrit kēsara, whose root 'kēsa' (hair) is Indo-European.

In Tibet, the existence of Gesar as a historical figure is rarely questioned (Samuel 1993, p. 365) (Li Lianrong 2001, p. 334). Some scholars have argued that he was born in 1027, based on a note in a 19th-century chronicle, the Mdo smad chos 'byung by Brag dgon pa dkon mchog bstan pa rab. Certain core episodes appear to reflect events recorded at the dawn of Tibetan history. For example, the marriage to a Chinese princess is reminiscent of legends concerning king Songtsän Gampo's alliance marriage with Princess Wencheng in 641. Legends variously place him in Golok, between Dotō and Domé, or in Markham, Tanak, Öyuk or the village of Panam on the Nyang River.