Dionysus is the god of the grape harvest, winemaking and wine, of ritual madness, fertility,theatre and religious ecstasy in Greek mythology. Wine played an important role in Greek culture with the cult of Dionysus the main religious focus for unrestrained consumption. His name, thought to be a theonym in Linear B tablets as di-wo-nu-so, shows that he may have been worshipped as early as c. 1500–1100 BC by Mycenean Greeks; other traces of the Dionysian-type cult have been found in ancient Minoan Crete. His origins are uncertain, and his cults took many forms; some are described by ancient sources as Thracian, others as Greek. In some cults, he arrives from the east, as an Asiatic foreigner; in others, from Ethiopia in the South. He is a god of epiphany, "the god that comes", and his "foreignness" as an arriving outsider-god may be inherent and essential to his cults. He is a major, popular figure of Greek mythologyand religion, becoming increasingly important over time, and is included in some lists of the twelve Olympians. Dionysus was the last god to be accepted into Mt. Olympus. He was the youngest and the only one to have a mortal mother. His festivals were the driving force behind the development of Greek theatre. He is sometimes categorised as a dying-and-rising god.

The earliest cult images of Dionysus show a mature male, bearded and robed. He holds a fennel staff, tipped with a pine-cone and known as a thyrsus. Later images show him as a beardless, sensuous, naked or half-naked androgynous youth: the literature describes him as womanly or "man-womanish". In its fully developed form, his central cult imagery shows his triumphant, disorderly arrival or return, as if from some place beyond the borders of the known and civilized. His procession (thiasus) is made up of wild female followers (maenads) and bearded satyrs with erect penises. Some are armed with the thyrsus, some dance or play music. The god himself is drawn in a chariot, usually by exotic beasts such as lions or tigers, and is sometimes attended by a bearded, drunken Silenus. This procession is presumed to be the cult model for the human followers of his Dionysian Mysteries. In his Thracian mysteries, he wears the bassaris or fox-skin, symbolizing a new life. Dionysus is represented by city religions as the protector of those who do not belong to conventional society and thus symbolizes everything which is chaotic, dangerous and unexpected, everything which escapes human reason and which can only be attributed to the unforeseeable action of the gods.

Also known as Bacchus, the name adopted by the Romans and the frenzy he induces, bakkheia. His thyrsus is sometimes wound with ivy and dripping with honey. It is a beneficent wand but also a weapon, and can be used to destroy those who oppose his cult and the freedoms he represents. He is also called Eleutherios ("the liberator"), whose wine, music and ecstatic dance frees his followers from self-conscious fear and care, and subverts the oppressive restraints of the powerful. Those who partake of his mysteries are possessed and empowered by the god himself. His cult is also a "cult of the souls"; his maenads feed the dead through blood-offerings, and he acts as a divine communicant between the living and the dead.

In Greek mythology, he is presented as a son of Zeus and the mortal Semele, thus semi-divine or heroic: and as son of Zeus and Persephoneor Demeter, thus both fully divine, part-chthonic and possibly identical with Iacchus of the Eleusinian Mysteries. Some scholars believe that Dionysus is a syncretism of a local Greek nature deity and a more powerful god from Thrace or Phrygia such as Sabazios or Zalmoxis.