In Greek mythology, Circe (/ˈsərsiː/; Ancient Greek: Κίρκη, romanized: Kírkē, pronounced [kírkɛː]) is an enchantress, sometimes considered a goddess or a nymph. In most accounts, Circe is described as the daughter of the sun god Helios and the Oceanid Perse. Circe was renowned for her vast knowledge of potions and herbs. Through the use of these and a magic wand or staff, she would transform her enemies, or those who offended her, into animals.

The best known of her legends is told in Homer's Odyssey when Odysseus visits her island of Aeaea on the way back from the Trojan War and she changes most of his crew into swine. He manages to persuade her to return them to human shape, lives with her for a year and has sons by her, including Latinus and Telegonus. Her ability to change others into animals is further highlighted by the story of Picus, an Italian king whom she turns into a woodpecker for resisting her advances. Another story tells of her falling in love with the sea-god Glaucus, who prefers the nymph Scylla to her. In revenge, Circe poisoned the water where her rival bathed and turned her into a dreadful monster.

Depictions, even in Classical times, diverged from the detail in Homer's narrative, which was later to be reinterpreted morally as a cautionary story against drunkenness. Early philosophical questions were also raised about whether the change from being a human endowed with reason to being an unreasoning beast might not be preferable after all, and the resulting debate was to have a powerful impact during the Renaissance. Circe was also taken as the archetype of the predatory female. In the eyes of those from a later age, this behaviour made her notorious both as a magician and as a type of sexually free woman. She has been frequently depicted as such in all the arts from the Renaissance down to modern times.

Western paintings established a visual iconography for the figure, but also went for inspiration to other stories concerning Circe that appear in Ovid's Metamorphoses. The episodes of Scylla and Picus added the vice of violent jealousy to her bad qualities and made her a figure of fear as well as of desire.

By most accounts, she was the daughter of the sun god Helios and Perse, one of the three thousand Oceanid nymphs. In Orphic Argonautica, her mother is called Asterope instead. Her brothers were Aeëtes, keeper of the Golden Fleece and father of Medea, and Perses. Her sister was Pasiphaë, the wife of King Minos and mother of the Minotaur. Other accounts make her and her niece Medea the daughters of Hecate, the goddess of witchcraft by Aeëtes, usually said to be her brother instead. She was often confused with Calypso, due to her shifts in behavior and personality, and the association that both of them had with Odysseus.

According to Greek legend, Circe lived on the island of Aeaea. Although Homer is vague when it comes to the island's whereabouts, the early 3rd BC author Apollonius of Rhodes's epic poem Argonautica locates Aeaea somewhere south of Aethalia (Elba), within view of the Tyrrhenian shore (that is, the western coast of Italy). In the same poem, Circe's brother Aeëtes describes how Circe was transferred to Aeaea: "I noted it once after taking a ride in my father Helios' chariot, when he was taking my sister Circe to the western land and we came to the coast of the Tyrrhenian mainland, where she dwells to this day, very far from the Colchian land." A scholiast on Apollonius Rhodius claims that Apollonius is following Hesiod's tradition in making Circe arrive in Aeaea on Helios' chariot, while Valerius Flaccus writes that Circe was borne away by winged dragons. Roman poets associated her with the most ancient traditions of Latium, and made her home to be on the promontory of Circeo.

Homer describes Circe as "a dreadful goddess with lovely hair and human speech". Apollonius writes that she (just like every other descendant of Helios) had flashing golden eyes that shot out rays of light, with the author of Argonautica Orphica noting that she had hair like fiery rays. Ovid's The Cure for Love implies that Circe might have been taught the knowledge of herbs and potions from her mother Perse, who seems to have had similar skills.

In the Argonautica, Apollonius relates that Circe purified the Argonauts for the murder of Medea's brother Absyrtus, possibly reflecting an early tradition. In this poem, the Argonauts find Circe bathing in salt water; the animals that surround her are not former lovers transformed but primeval "beasts, not resembling the beasts of the wild, nor yet like men in body, but with a medley of limbs." Circe invites Jason, Medea and their crew into her mansion; uttering no words, they show her the still bloody sword they used to cut Absyrtus down, and Circe immediately realizes they have visited her to be purified of murder. She purifies them by slitting the throat of a suckling pig and letting the blood drip on them. Afterwards, Medea tells Circe their tale in great detail, albeit omitting the part of Absyrtus' murder; nevertheless Circe is not fooled, and greatly disapproves of their actions. However, out of pity for the girl, and on account of their kinship, she promises not to be an obstacle on their way, and orders Jason and Medea to leave her island immediately.