Gilgamesh (Sumerian: 𒀭𒄑𒉋𒂵𒈨𒌋𒌋𒌋, romanized: Gilgameš; originally Sumerian: 𒀭𒉋𒂵𒈩, romanized: Bilgamesh)^[a] was a major hero in ancient Mesopotamian mythology and the protagonist of the Epic of Gilgamesh, an epic poem written in Akkadian during the late 2nd millennium BC. He was also most likely a historical king of the Sumerian city-state of Uruk, who was posthumously deified. His rule probably would have taken place sometime between 2800 and 2500 BC, though he became a major figure in Sumerian legend during the Third Dynasty of Ur (c. 2112 – c. 2004 BC).

Tales of Gilgamesh's legendary exploits are narrated in five surviving Sumerian poems. The earliest of these is most likely "Gilgamesh, Enkidu, and the Netherworld", in which Gilgamesh comes to the aid of the goddess Inanna and drives away the creatures infesting her huluppu tree. She gives him two unknown objects, a mikku and a pikku, which he loses. After Enkidu's death, his shade tells Gilgamesh about the bleak conditions in the Underworld. The poem "Gilgamesh and Agga" describes Gilgamesh's revolt against his overlord King Agga. Other Sumerian poems relate Gilgamesh's defeat of the ogre Huwawa and the Bull of Heaven, while a fifth, poorly preserved poem apparently describes his death and funeral.

In later Babylonian times, these stories began to be woven into a connected narrative. The standard Akkadian Epic of Gilgamesh was composed by a scribe named Sîn-lēqi-unninni, probably during the Middle Babylonian Period (c. 1600 – c. 1155), based on much older source material. In the epic, Gilgamesh is a demigod of superhuman strength who befriends the wildman Enkidu. Together, they go on adventures, defeating Humbaba (Sumerian: Huwawa) and the Bull of Heaven, who is sent to attack them by Ishtar (Sumerian: Inanna) after Gilgamesh rejects her offer for him to become her consort. After Enkidu dies of a disease sent as punishment from the gods, Gilgamesh becomes afraid of his own death, and visits the sage Utnapishtim, the survivor of the Great Flood, hoping to find immortality. Gilgamesh repeatedly fails the trials set before him and returns home to Uruk, realizing that immortality is beyond his reach.

Most classical historians agree that the Epic of Gilgamesh exerted substantial influence on both the Iliad and the Odyssey, two epic poems written in ancient Greek during the 8th century BC. The story of Gilgamesh's birth is described in an anecdote from On the Nature of Animals by the Greek writer Aelian (2nd century AD). Aelian relates that Gilgamesh's grandfather kept his mother under guard to prevent her from becoming pregnant, because he had been told by an oracle that his grandson would overthrow him. She became pregnant and the guards threw the child off a tower, but an eagle rescued him mid-fall and delivered him safely to an orchard, where he was raised by the gardener.

The Epic of Gilgamesh was rediscovered in the Library of Ashurbanipal in 1849. After being translated in the early 1870s, it caused widespread controversy due to similarities between portions of it and the Hebrew Bible. Gilgamesh remained mostly obscure until the mid-20th century, but, since the late 20th century, he has become an increasingly prominent figure in modern culture.