In Greek mythology, Caeneus or Kaineus (Ancient Greek: Καινεύς, romanized: Kaineús) was a Lapith hero, ruler of Thessaly, and the father of the Argonaut Coronus. Caeneus was born a girl, Caenis (Ancient Greek: Καινίς, romanized: Kainís), the daughter of Elatus, but after Poseidon had sex with Caenis, she was transformed by him into an invulnerable man. Caeneus participated in the Centauromachy, where he met his demise at the hands of the Centaurs by being pounded into the ground while still alive.

Caeneus' father was the Lapith king Elatus, from Gyrton in Thessaly. Caeneus' son was the Argonaut Coronus, who was killed by Heracles while leading a war against the Dorians and their king Aegimius. According to the mythographer Hyginus, Caeneus' mother was Hippea—the daughter of a Thessalian from Larissa named Antippus—and his brothers were Ischys and the Argonaut Polyphemus. Hyginus also states that, in addition to Coronus, Caeneus had two other sons: Phocus and Priasus, who were also Argonauts. According to Antoninus Liberalis, his father was Atrax, rather than Elatus.

Caeneus was originally a woman named Caenis who was transformed into a man by the sea-god Poseidon. Although possibly as old as the Hesiodic Catalogue of Women (c. first half of the sixth century BC), the oldest secure mention of this transformation comes from the mythographer Acusilaus (sixth to fifth century BC). According to Acusilaus, after having sex with Poseidon, Elatus' daughter—here instead called Caene—did not want to have a child by Poseidon or anyone else, due to an unspecified vow or prohibition against it; to prevent this, Poseidon transformed Caene into an invulnerable man, stronger than any other. However, according to the usual version of events, after having sex with Caenis, Poseidon promised he would do whatever Caenis wanted, so Caenis asked to be transformed into an invulnerable man, which Poseidon did.

Besides the Centauromachy, little is said about Caeneus' activities after his transformation. According to Acusilaus, Caeneus was the strongest warrior of his day, and became king of the Lapiths. While king, Caeneus angered the gods by an act of impiety, although accounts differ; according to an Iliad scholiast, Caeneus set up his spear in the agora and ordered his subjects to worship it, while according to a scholiast on Apollonius of Rhodes' Argonautica, Caeneus himself worshipped his spear rather than the gods. In either case, Caeneus' actions so offended the gods that, as Acusilaus goes on to say, Zeus sent the Centaurs against him. The Oxyrhynchus Papyrus that supplies Acusilaus' account says that Caeneus was used by Theophrastos as an example of ruling by the "spear" rather than the "scepter"—that is, by force rather than authority.

Caeneus was also listed as among those who took part in the Calydonian boar hunt by the sixth-century BC Greek lyric poet Stesichorus, as well as by the Roman poet Ovid and the Roman mythographer Hyginus, although no details of his participation are given.

Caeneus' participation in the Centauromachy—the battle between the Lapiths and the Centaurs at the wedding feast of Pirithous—seems to be the earliest story told about Caeneus. His transformation and other stories are likely later elaborations.

Caeneus fought in the Centauromachy, where most accounts say he met his demise. Because of his invulnerability, none of the Centaurs' weapons could hurt him, so in order to defeat the Lapith king, they hammered him into the ground with tree trunks and boulders, which succeeded in restraining him alive.

Caeneus' earliest mention occurs in Homer's Iliad, where Nestor names Caeneus among those "mightiest" of warriors who fought and defeated the Centaurs: