The Eleusinian Mysteries (Greek: Ἐλευσίνια Μυστήρια, romanized: Eleusínia Mystḗria) were initiations held every year for the cult of Demeter and Persephone based at the Panhellenic Sanctuary of Eleusis in ancient Greece. They are considered the "most famous of the secret religious rites of ancient Greece". Their basis was an old agrarian cult, and there is some evidence that they were derived from the religious practices of the Mycenean period. The Mysteries represented the myth of the abduction of Persephone from her mother Demeter by the king of the underworld Hades, in a cycle with three phases: the descent (loss), the search, and the ascent, with the main theme being the ascent (ἄνοδος) of Persephone and the reunion with her mother. It was a major festival during the Hellenic era, and later spread to Rome. Similar religious rites appear in the agricultural societies of the Near East and in Minoan Crete.

The rites, ceremonies, and beliefs were kept secret and consistently preserved from antiquity. For the initiated, the rebirth of Persephone symbolized the eternity of life which flows from generation to generation, and they believed that they would have a reward in the afterlife. There are many paintings and pieces of pottery that depict various aspects of the Mysteries. Since the Mysteries involved visions and conjuring of an afterlife, some scholars believe that the power and longevity of the Eleusinian Mysteries, a consistent set of rites, ceremonies and experiences that spanned two millennia, came from psychedelic drugs. The name of the town, Eleusis, seems to be pre-Greek, and is likely a counterpart with Elysium and the goddess Eileithyia.