In ancient Greek religion and myth, Dionysus (Ancient Greek: Διόνυσος, Diónysos) is the god of wine-making, orchards, fruit, vegetation, fertility, festivity, insanity, ritual madness, religious ecstasy, and theatre. He was also known as Bacchus (Ancient Greek: Βάκχος, Bacchos) by the Greeks — a name later adopted by the Romans — for a frenzy he is said to induce called baccheia. His wine, music, and ecstatic dance were considered to free his followers from self-conscious fear and care, and subvert the oppressive restraints of the powerful. His thyrsus, a fennel-stem sceptre, sometimes wound with ivy and dripping with honey, is both a beneficent wand and a weapon used to destroy those who oppose his cult and the freedoms he represents. Those who partook of his mysteries were believed to become possessed and empowered by the god himself. His origins are uncertain, and his cults took many forms. Traditionally the cult of Dionysus was said to have come from Asia Minor and to have been introduced to the Greeks via Thrace; but his name appears in Linear B tablets from Pylos, so he may be of Mycenaean origin.

Academics in the nineteenth century, using study of philology and comparative mythology, often regarded Dionysus as a foreign deity who was only reluctantly accepted into the standard Greek pantheon at a relatively late date, based on his myths which often involve this theme—a god who spends much of his time on earth abroad, and struggles for acceptance when he returns to Greece. However, more recent evidence has shown that Dionysus was in fact one of the earliest gods attested in mainland Greek culture. The earliest written records of Dionysus worship come from Mycenaean Greece, specifically in and around the Palace of Nestor in Pylos, dated to around 1300 BC. The details of any religion surrounding Dionysus in this period are scant, and most evidence comes in the form only of his name, written as di-wo-nu-su-jo ("Dionysoio" = 'of Dionysus') in Linear B, preserved on fragments of clay tablets that indicate a connection to offerings or payments of wine, which was described as being "of Dionysus". References have also been uncovered to "women of Oinoa", the "place of wine", who may correspond to the Dionysian women of later periods. Some 19th-century classicists such as Matthew Arnold identified Phanes as a prototype for Dionysus, and argued that he was originally venerated not as a bringer of wine and revelry but as a "custodian of borrowed identities".

Various different accounts and traditions existed in the ancient world regarding the parentage, birth, and life of Dionysus on earth, complicated by his several rebirths. By the first century BC, some mythographers had attempted to harmonise the various accounts of Dionysus's birth into a single narrative involving not only multiple births, but two or three distinct manifestations of the god on earth throughout history in different lifetimes. The historian Diodorus Siculus said that according to "some writers of myths" there were two gods named Dionysus, an older one, who was the son of Zeus and Persephone, but that the "younger one also inherited the deeds of the older, and so the men of later times, being unaware of the truth and being deceived because of the identity of their names thought there had been but one Dionysus." He also said that Dionysus "was thought to have two forms ... the ancient one having a long beard, because all men in early times wore long beards, and the younger one being long-haired, youthful and effeminate and young."

Though the varying genealogy of Dionysus was mentioned in many works of classical literature, only a few contain the actual narrative myths surrounding the events of his multiple births. These include the first century BC Bibliotheca historica by Greek historian Diodorus, which describes the birth and deeds of the three incarnations of Dionysus; the brief birth narrative given by the first century AD Roman author Hyginus, which describes a double birth for Dionysus; and a longer account in the form of Greek poet Nonnus's epic Dionysiaca, which discusses three incarnations of Dionysus similar to Diodorus's account, but which focuses on the life of the third Dionysus, born to Zeus and Semele.

Though Diodorus mentions some traditions which state an older, Indian or Egyptian Dionysus existed who invented wine, no narratives are given of his birth or life among mortals, and most traditions ascribe the invention of wine and travels through India to the last Dionysus. According to Diodorus, Dionysus was originally the son of Zeus and Persephone (or alternately, Zeus and Demeter). This is the same horned Dionysus described by Hyginus and Nonnus in later accounts, and the Dionysus worshiped by the Orphics, who was dismembered by the Titans and then reborn. Nonnus calls this Dionysus Zagreus, while Diodorus says he is also considered identical with Sabazius. However, unlike Hyginus and Nonnus, Diodorus does not provide a birth narrative for this incarnation of the god. It was this Dionysus who was said to have taught mortals how to use oxen to plow the fields, rather than doing so by hand. His worshipers were said to have honored him for this by depicting him with horns.

The worship of Dionysus had become firmly established by the seventh century BC. He may have been worshiped as early as c. 1500–1100 BC by Mycenaean Greeks; and traces of Dionysian-type cult have also been found in ancient Minoan Crete.

The Dionysia, Haloa, Ascolia and Lenaia festivals were dedicated to Dionysus. The Rural Dionysia (or Lesser Dionysia) was one of the oldest festivals dedicated to Dionysus, begun in Attica, and probably celebrated the cultivation of wines. It was held during the winter month of Poseideon (the time surrounding the winter solstice, modern December or January). The Rural Dionysia centered on a procession, during which participants carried phalluses, long loaves of bread, jars of water and wine as well as other offerings, and young girls carried baskets. The procession was followed by a series of dramatic performances and drama competitions. The City Dionysia (or Greater Dionysia) took place in urban centers such as Athens and Eleusis, and was a later development, probably beginning during the sixth century BC. Held three months after the Rural Dionysia, the Greater festival fell near the spring equinox in the month of Elaphebolion (modern March or April).