In ancient Greek religion and mythology, Demeter (; Attic: Δημήτηρ Dēmḗtēr [dɛːmɛ́ːtɛːr]; Doric: Δαμάτηρ Dāmā́tēr) is the Olympian goddess of the harvest and agriculture, presiding over crops, grains, food, and the fertility of the earth. Although she is mostly known as a grain goddess, she also appeared as a goddess of health, birth, and marriage, and had connections to the Underworld. She is also called Deo (Δηώ). In Greek tradition, Demeter is the second child of the Titans Rhea and Cronus, and sister to Hestia, Hera, Hades, Poseidon, and Zeus. Like her other siblings but Zeus, she was swallowed by her father as an infant and rescued by Zeus.

Through her brother Zeus, she became the mother of Persephone, a fertility goddess. One of the most notable Homeric Hymns, the Homeric Hymn to Demeter, tells the story of Persephone's abduction by Hades and Demeter's search for her. When Hades, the King of the Underworld, wished to make Persephone his wife, he abducted her from a field while she was picking flowers, with Zeus' leave. Demeter searched everywhere to find her missing daughter to no avail until she was informed that Hades had taken her to the Underworld. In response, Demeter neglected her duties as goddess of agriculture, plunging the earth into a deadly famine where nothing would grow, causing mortals to die. Zeus ordered Hades to return Persephone to her mother to avert the disaster. However, because Persephone had eaten food from the Underworld, she could not stay with Demeter forever but had to divide the year between her mother and her husband, explaining the seasonal cycle, as Demeter does not let plants grow while Persephone is gone.

Her cult titles include Sito (Σιτώ), "she of the Grain", as the giver of food or grain, and Thesmophoros (θεσμός, thesmos: divine order, unwritten law; φόρος, phoros: bringer, bearer), "giver of customs" or "legislator", in association with the secret female-only festival called the Thesmophoria. Though Demeter is often described simply as the goddess of the harvest, she presided also over the sacred law, and the cycle of life and death. She and her daughter Persephone were the central figures of the Eleusinian Mysteries, a religious tradition that predated the Olympian pantheon and which may have its roots in the Mycenaean period c. 1400–1200 BC.

Demeter was often considered to be the same figure as the Anatolian goddess Cybele, and she was identified with the Roman goddess Ceres.