In Greek mythology, Ismene (/ɪsˈmiːniː/; Ancient Greek: Ἰσμήνη, romanized: Ismḗnē) was a Theban princess. She was the daughter and half-sister of Oedipus, king of Thebes, daughter and granddaughter of Jocasta, and sister of Antigone, Eteocles, and Polynices. She appears in several tragic plays of Sophocles, including Oedipus Rex, Oedipus at Colonus, and Antigone. She also appears at the end of Aeschylus' Seven Against Thebes.

Traditionally, Ismene was the daughter of Oedipus and Jocasta (Epicaste), king and queen of Thebes. She had three full siblings: Antigone, Polynices, and Eteocles, and her father Oedipus was also her half-brother.

In the lost Oedipodea, the archaic epic poem that narrated the adventures of Oedipus and inspired later authors, the children of Oedipus were mothered by Euryganeia, daughter of Hyperphas. In the Odyssey, Homer writes that, after Oedipus' incestuous marriage to his mother, the gods revealed the incest "straightaway." In reference to this, Pausanias claims that the notion that the gods revealed it straightaway is incompatible with the idea that Oedipus and Jocasta had four children, so his children must have been born to another woman.

There is no absolute confirmation that Ismene was a part of the Oedipodea at all, let alone as Oedipus' daughter. Ismene was likely a separate Theban heroine integrated into Oedipus' myth and family tree by the tragedians during the fifth century BC.

The seventh-century BC poet Mimnermus wrote that Ismene was murdered by Tydeus, one of the Seven who took arms against Thebes. In this account, Ismene was dallying with her lover Periclymenus (Theoclymenus) when Tydeus surprised them, having being told of their whereabouts by the goddess Athena. Tydeus then slew Ismene. It is not clear what Athena's motivation was; perhaps here Ismene was a priestess of Athena, so the goddess was punishing her transgression. This tradition cannot be reconciled with the events depicted in later works by Aeschylus and Sophocles, in which Ismene appears alive after the battle is over and Tydeus is dead; it is also incompatible with Ismene's later timid and meek characterization.

This version of the story is mentioned in no other extant classical writing, but the scene is represented on a sixth-century BC Corinthian black-figure amphora now housed in the Louvre. The vase depicts Tydeus brandishing his sword at Ismene (spelled Hysmena), who is nude and reclining on a couch, while her lover Periclymenus flees and looks back, not at Ismene, but at Tydeus's sword. The scene probably takes place in a home or palace in Thebes. As Periclymenus is said to be the killer of Parthenopaeus, one of the Seven, the vase might depict a tradition in which Tydeus infiltrated Thebes, found Perclymenus and Ismene together in bed, and slew Ismene in revenge while her lover/husband managed to escape.

In another tradition by the early fifth-century poet Ion of Chios, Eteocles's son Laodamas burned Ismene alive together with Antigone inside Hera's temple.

Ismene is first identified as Oedipus and Jocasta's daughter by Aeschylus. His play Seven Against Thebes depicts the war and demise of Eteocles and Polynices. At the end of the play, the chorus narrates Ismene and Antigone entering to sing a funeral dirge together for both of their brothers. While Antigone exits with the first semichorus, escorting the body of Polynices, Ismene and the second semichorus exit with the body of Eteocles.