In ancient Greek religion and mythology, Iacchus was a minor deity, of some cultic importance, particularly at Athens and Eleusis in connection with the Eleusinian mysteries, but without any significant mythology. He perhaps originated as the personification of the ritual exclamation Iacche! cried out during the Eleusinian procession from Athens to Eleusis. He was often identified with Dionysus, perhaps because of the resemblance of the names Iacchus and Bacchus, another name for Dionysus. By various accounts he was a son of Demeter, or a son of Persephone, identical with Dionysus Zagreus, or a son of Dionysus.

The Eleusinian Mysteries 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 a Bronze Age 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 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.