The Bacchae (/ˈbækiː/; Ancient Greek: Βάκχαι, Bakkhai; also known as The Bacchantes /ˈbækənts, bəˈkænts, -ˈkɑːnts/) is an ancient Greek tragedy, written by the Athenian playwright Euripides during his final years in Macedonia, at the court of Archelaus I of Macedon. It premiered posthumously at the Theatre of Dionysus in 405 BC as part of a tetralogy that also included Iphigeneia at Aulis and Alcmaeon in Corinth, and which Euripides' son or nephew is assumed to have directed. It won first prize in the City Dionysia festival competition.

The tragedy recounts the Greek myth of King Pentheus of Thebes and his mother Agave, who were punished by the god Dionysus (who is Pentheus's cousin) for rejecting his cult. The play opens with Dionysus proclaiming that he has arrived in Thebes with his votaries to avenge the slander, repeated by his aunts, that he is not the son of Zeus. Disguised as a foreign holy man, the god intends to introduce Dionysian rites into the city, but the Thebans reject his divinity and king Pentheus orders his arrest.

Eventually, Dionysus drives Pentheus insane, luring him to the mountains. The play ends with the women of Thebes, driven by Dionysus's orgiastic frenzy, tearing Pentheus apart, while his mother Agave bears his head on a thyrsus to her father Cadmus.

Regarded as Euripides' masterpiece, The Bacchae is classified among the greatest ancient tragedies. The Bacchae is distinctive in that the chorus is integrated into the plot and the god is not a distant presence, but a character in the play, indeed, the protagonist.

The Dionysus in Euripides' tale is a young god, angry that his mortal family, the royal house of Cadmus, has denied him a place of honor as a deity. His mortal mother, Semele, was a mistress of Zeus; and while pregnant, was tricked by a jealous Hera to request Zeus to come to her in his true form. Being only a mortal, she was struck down by Zeus' thunderbolts while in his presence and was killed.

Zeus then saved Dionysus, who was in Semele's womb, by sewing him into a cavity in his thigh. When Semele died, her sisters said it was Zeus' will and accused her of lying; they also accused their father, Cadmus, of claiming Semele was pregnant by Zeus to cover up an affair with a mortal man. Most of Semele's family refused to believe Dionysus was the son of Zeus, and the young god was spurned by his household. He traveled throughout Asia and other foreign lands, gathering a cult of female worshipers, the Maenads.

At the start of the play, Dionysus returns to Thebes, disguised as a stranger, to take revenge on the house of Cadmus. He has also driven the women of Thebes, including his aunts, into an ecstatic frenzy, sending them dancing and hunting on Mount Cithaeron, much to the horror of the young Pentheus, king of Thebes who also is Dionysus' cousin. Complicating matters, Pentheus has declared a ban on the worship of Dionysus throughout Thebes.

The play begins before the palace at Thebes, with Dionysus telling the story of his birth and his reasons for visiting the city. Dionysus explains he is the son of a mortal woman, Semele, and a god, Zeus. Some in Thebes, he notes, do not believe this story. In fact, Semele's sisters—Autonoe, Agave, and Ino—claim it is a lie intended to cover up the fact that Semele became pregnant by some mortal. Dionysus reveals that he has driven the women of the city mad, including his three aunts, and has led them into the mountains to observe his ritual festivities. He has disguised himself as a mortal for the time being, but he plans to vindicate his mother by appearing before all of Thebes as a god, the son of Zeus, and establishing his permanent cult of followers.