Spartan king from 519 BC to his death, a ruler who consolidated his city's position as the leading power in the Peloponnesus. He refused to commit Spartan forces overseas against the Persians but readily intervened in the affairs of his Greek rival, Athens. A member of the Agiad house, he succeeded his father Anaxandridas as king.

In 510 he led an army to Athens and expelled the tyrant Hippias. Three years later at Athens he supported Isagoras, the leader of the oligarchs, against the democratic Cleisthenes. Both Cleomenes and Isagoras, however, were forced to withdraw. Next Cleomenes raised a large Peloponnesian army for use against Athens, but the enterprise failed after the Corinthian contingent mutinied with the support of the other Spartan king, Demaratus. Cleomenes rejected Miletus' appeal (499) for aid in the revolt of the Ionian Greeks against Persia. In 494 he inflicted a severe defeat on Argos at Sepeia near Tiryns. Three years later he tried to punish Aegina for its submission to the Persians, but Demaratus again thwarted him. Cleomenes engineered the deposing of Demaratus by bribing the Delphic oracle, but his deceit was discovered, and he fled to Thessaly. The Spartans reinstated him, but soon afterwards he went insane and committed suicide.