Megistonus or Megistonous (gr. Μεγιστόνους), a Spartan of rank and influence, whom Cratesicleia, the mother of Cleomenes III., took for her second husband, with the view, as it would seem, of securing him to her son's party; and we find him accordingly entering readily into the plans of Cleomenes for the reformation of the state. In 226 BC he was taken prisoner by Aratus in a battle near Orchomenus in Arcadia; but he must have been soon released, for he appears again not long after at Sparta, co-operating with Cleomenes in the measures which he proposed after the murder of the Ephori, and setting an example to his countrymen by the voluntary surrender of his property. In 223 BC, when Cleomenes took Argos, Megistonous induced him to adopt no steps against those citizens who were suspected of an attachment to the Achaean party, beyond the requisition of twenty hostages. In the same year Cleomenes, having taken possession of Corinth, and besieged the citadel, sent Megistonons and Triplus, or Tritymallus, to Aratus, then at Sicyon, with an offer of terms, which, however, were rejected. Not long after this, the Achaean party in Argos excited an insurrection against the Spartan garrison ; and Megistonous, being sent by Cleomenes with 2000 men to quell the revolt, was slain in battle soon after he had thrown himself into the city. (Plut. Cleom. 6, 7, 11, 19, 21, Arat. 38,41, 44; comp. Polyb. ii. 47, 52, 53; Droysen, Hellenismus, vol. ii. b. ii. ch. 4.)