Megistias (gr. Μεγιστίας), a celebrated soothsayer, a native of Acarnania, who traced his descent up to Melampus. He was present at the battle of Thermopylae; and though he foresaw by his art the fatal issue of the conflict, refused to quit his post, though requested to withdraw by Leonidas. He sent away his only son, but himself remained and was killed. A separate monument was erected to his memory with an inscription by his friend Simonides, which is quoted by Herodotus. (Herod. vii. 219, 221, 228.) Plutarch (Apophth. Lacon. vol. ii. p. 221, e.) gives the name Themisteas to the soothsayer whom Leonidas wished to send away.