Ceto (Ancient Greek: Κητώ, Kētō, "sea monster") is a primordial sea goddess in Greek mythology, the daughter of Gaia and Pontus. Ceto was also variously called Crataeis (Κράταιις, Krataiis, from κραταιίς "mighty") and Trienus (Τρίενος, Trienos, from τρίενος "within three years"), and was occasionally conflated by scholars with the goddess Hecate (for whom Trienus and Crataeis are also epithets). As a mythological figure, she is most notable for bearing by Phorcys a host of monstrous children, collectively known as the Phorcydes. The asteroid 65489 Ceto was named after her, and its satellite after Phorcys.

This goddess should not be confused with the minor Oceanid also named Ceto — who appears in Hesiod's Theogony as a separate character from Ceto the daughter of Pontus and Gaia — or with various mythological beings referred to as ketos (plural ketea); this is a general term for "sea monster" in Ancient Greek.

Hesiod's Theogony lists the children of Phorcys and Ceto as Echidna, The Gorgons (Euryale, Stheno, and the infamous Medusa), The Graeae (Deino, Enyo, and Pemphredo), and Ladon, also called the Drakon Hesperios ("Hesperian Dragon", or dragon of the Hesperides). These children tend to be consistent across sources, though Ladon is sometimes cited as a child of Echidna by Typhon and therefore Phorcys and Ceto's grandson.