In Greek mythology, Thestius was the son of either Ares and Demonice, or Agenor (son of Pleuron son of Aetolus) by Epicasta. He was the father of Iphiclus with Leucippe (or, if Ares was his father, with Eurythemis). With Eurythemis, he may have also been the father of Althea, Eurypylus, Evippus, Hypermnestra, Leda and Plexippus. He was allied with Tyndareus and Icarius.
Not to be confused with Thespius, who was sometimes referred to as "Thestius."
Thestius became king of Pleuron while Oeneus, the son of Porthaon, became king of Calydon.
Althaea became the wife of her uncle Oeneus and the mother of Meleagrus and Deiareina (one of Heracles' wives), while Leda became the wife of Tyndareus, the king of Sparta, while he was in exile at the court of her father after having been unseated by his half-brother Hippocoon.
Thestius was also represented as a descendant of Pleuron; and hence Pleuron had an heroum or a chapel at Sparta, as being the ancestor of Leda, the daughter of Thestius.