In Greek mythology, Andromeda was the daughter of Cepheus and Cassiopeia, king and queen of the kingdom Aethiopia.
Her mother Cassiopeia boasted that her daughter was more beautiful than the Nereids, the nymph-daughters of the sea god Nereus and often seen accompanying Poseidon. To punish the queen for her arrogance, Poseidon, brother to Zeus and god of the sea, sent a sea monster named Cetus to ravage the coast of Aethiopia including the kingdom of the vain queen. The desperate king consulted the Oracle of Apollo, who announced that no respite would be found until the king sacrificed his daughter, Andromeda, to the monster. Stripped naked, she was chained to a rock on the coast.
Perseus was returning from having slain the Gorgon Medusa. After he happened upon the chained Andromeda, he approached Cetus while invisible (for he was wearing Hades's helm), and killed the sea monster. He set Andromeda free, and married her in spite of her having been previously promised to her uncle Phineus. At the wedding a quarrel took place between the rivals and Phineus was turned to stone by the sight of the Gorgon's head.
Andromeda followed her husband, first to his native island of Serifos, where he rescued his mother Danaë, and then to Tiryns in Argos. Together, they became the ancestors of the family of the Perseidae through the line of their son Perses. Perseus and Andromeda had seven sons: Perses, Alcaeus, Heleus, Mestor, Sthenelus, and Electryon, as well as two daughters, Autochthe and Gorgophone. Their descendants ruled Mycenae from Electryon down to Eurystheus, after whom Atreus attained the kingdom, and would also include the great hero Heracles. According to this mythology, Perseus is the ancestor of the Persians.
At the port city of Jaffa (today part of Tel Aviv) an outcrop of rocks near the harbour has been associated with the place of Andromeda's chaining and rescue by the traveler Pausanias, the geographer Strabo and the historian of the Jews Josephus.
After Andromeda's death, as Euripides had promised Athena at the end of his Andromeda, produced in 412 BCE, the goddess placed her among the constellations in the northern sky, near Perseus and Cassiopeia; the constellation Andromeda, so known since antiquity, is named after her.