Clytemnestra (also Klytaimnéstra or Clytaemnestra, "praiseworthy wooing") was the wife of Agamemnon, king of the Greek kingdom of Mycenae or Argos. She is the daughter of Tyndareus and Leda and mother of Iphigeneia, Orestes, Chrysothemis and Electra. She is also believed to have been born of a union between Zeus and Leda, the former having wooed the latter in the guise of a swan. According to legend, following her union with the Olympian, Leda laid two eggs, Castor and Polydeuces (the Dioscuri, also known as the constellation Gemini) were hatched from one, and Helen (later of Troy) and Clytemnestra from the other.

Agamemnon followed his brother Menelaus after Menelaus' wife Helen was stolen by Paris, thus igniting the Trojan War.

While Agamemnon was away, Clytemnestra weakened her resolve and began a torrid love affair with Aegisthus, her husband's kinsman (daughter with Aegisthus: Erigone). She was bitter towards her absent husband for having sacrificed their daughter, Iphigeneia, to Artemis.

At the end of the war, Agamemnon returned to Mycenae where his kinsman, Aegisthus, who in the interval had seduced his wife Clytemnestra, invited him to a banquet at which he was treacherously slain. Princess Cassandra of Troy, who had been taken by Agamemnon as a war trophy, was also put to death by Clytemnestra. According to the account given by Pindar and the tragedians, Agamemnon was slain by his wife alone in a bath, a piece of cloth or a net having first been thrown over him to prevent resistance. According to Aeschylus, Clytemnestra placed a piece of purple cloth and asked the returning Agamemnon to step over it. He refused at first but then gave in, while Cassandra, who had been endowed with the gift of prophecy but with the curse of no one believing her, waited outside, knowing doom awaited. She stayed outside until she heard Agamemnon scream as he died, then ran inside and was killed by Clytemnestra. Clytemnestra's wrath at the sacrifice of their daughter Iphigeneia, and her jealousy of Cassandra, are said to have been the motives of her crime. The murder of Agamemnon was avenged by his son Orestes.

The story of Orestes is told in Aeschylus's famous trilogy, the Oresteia, in Sophocles's play Electra, and in Euripides's play Electra.