In Greek mythology, Gaia (/ˈɡeɪə, ˈɡaɪə/;] from Ancient Greek Γαῖα, a poetical form of Γῆ Gē, "land" or "earth"), also spelled Gaea /ˈdʒiːə/, is the personification of the Earth and one of the Greek primordial deities. Gaia is the ancestral mother—sometimes parthenogenic—of all life. She is the mother of Uranus (the sky), from whose sexual union she bore the Titans (themselves parents of many of the Olympian gods), the Cyclopes, and the Giants; as well as of Pontus (the sea), from whose union she bore the primordial sea gods. Her equivalent in the Roman pantheon was Terra.

Hesiod's Theogony tells how, after Chaos, "wide-bosomed" Gaia (Earth) arose to be the everlasting seat of the immortals who possess Olympus above. And after Gaia came "dim Tartarus in the depth of the wide-pathed Earth", and next Eros the god of love. Hesiod goes on to say that Gaia brought forth her equal Uranus (Heaven, Sky) to "cover her on every side". Gaia also bore the Ourea (Mountains), and Pontus (Sea), "without sweet union of love" (i.e., with no father).

Afterwards with Uranus, her son, she gave birth to the Titans, as Hesiod tells it:

She lay with Heaven and bore deep-swirling Oceanus, Coeus and Crius and Hyperion and Iapetus, Theia and Rhea, Themis and Mnemosyne and gold-crowned Phoebe and lovely Tethys. After them was born Cronos (Cronus) the wily, youngest and most terrible of her children, and he hated his lusty sire.

According to Hesiod, Gaia conceived further offspring with her son, Uranus, first the giant one-eyed Cyclopes: Brontes ("Thunder"), Steropes ("Lightning"), and Arges ("Bright"); then the Hecatonchires: Cottus, Briareos, and Gyges, each with a hundred arms and fifty heads. As each of the Cyclopes and Hecatonchires were born, Uranus hid them in a secret place within Gaia, causing her great pain. So Gaia devised a plan. She created a grey flint (or adamantine) sickle. And Cronus used the sickle to castrate his father Uranus as he approached his mother, Gaia, to have sex with her. From Uranus' spilled blood, Gaia produced the Erinyes, the Giants, and the Meliae (ash-tree nymphs). From the testicles of Uranus in the sea came forth Aphrodite.

By her son, Pontus, Gaia bore the sea-deities Nereus, Thaumas, Phorcys, Ceto, and Eurybia.

Because Cronus had learned from Gaia and Uranus that he was destined to be overthrown by one of his children, he swallowed each of the children born to him by his Titan older sister, Rhea. But when Rhea was pregnant with her youngest child, Zeus, she sought help from Gaia and Uranus. When Zeus was born, Rhea gave Cronus a stone wrapped in swaddling-clothes in his place, which Cronus swallowed, and Gaia took the child into her care.

With the help of Gaia's advice, Zeus defeated the Titans. But afterwards, Gaia, in union with Tartarus, bore the youngest of her sons Typhon, who would be the last challenge to the authority of Zeus.