Cleomenes was the King of Sparta in the 6th and 5th centuries BC. He was the son of Anaxandrides and his second wife, and the half-brother of Dorieus. Although Dorieus was the son of Anaxandrides' first wife and therefore had a better claim to the throne according to tradition, Cleomenes succeeded his father around 520 BC.

Around 510 BC the Alcmaeonidae family, who had been exiled from Athens, requested that Sparta help them overthrow Hippias, the son of Pisistratus and tyrant of Athens. The Alcmaeonidae, led by Cleisthenes, bribed the oracle at Delphi to tell the Spartans to assist them, and Cleomenes came to their aid. The first attack on Athens was a failure, but Cleomenes personally led the second attack and besieged Hippias and his supporters on the Acropolis. He was unable to force Hippias to surrender, but the Spartans captured some of Hippias' relatives and took them hostage until he agreed to give up the city. but later attempted to restore Hippias as tyrant when the Spartans realized Athens was too powerful for them to control.

Cleisthenes and Isagoras then fought for control of Athens. Cleomenes supported Isagoras and they forced Cleisthenes and the Alcmaeonidae family to go into exile for a second time. Cleomenes also abolished the Boule, a council set up by Cleisthenes, and occupied the Acropolis. The citizens of Athens objected to this and forced him out of the city. Cleomenes gathered an army, intending to set up Isagoras as tyrant, and invaded Attica. The Corinthians in his force refused to attack Athens once they learned of Cleomenes' plan, and the invasion failed.

Cleomenes was still king when Aristagoras, the tyrant of Miletus, came to Sparta to request help for the Ionian Revolt in 499 BC. Aristagoras was almost able to convince Cleomenes to help, promising an easy conquest of Persia and its riches, but Cleomenes sent him away when he learned how far away Persia really was.

When the Persians invaded Greece after putting down the revolt in 494 BC, many city-states quickly submitted to them. Among these states was Aegina, so Cleomenes attempted to arrest the major collaborators there. The Aeginetans would not cooperate with him, and the other Spartan king, Demaratus, was also attempting to undermine his efforts. Cleomenes overthrew Demaratus, after first bribing the oracle at Delphi to announce that this was the divine will, and replaced him with Leotychides. The two kings successfully captured the Persian collaborators in Aegina.

Also around 494, Cleomenes invaded Argos, and by fooling the Argive army he killed about 6000 inhabitants. Argos remained a bitter enemy of Sparta for decades after this attack.

Around 490 BC Cleomenes was forced to flee Sparta when his plot against Demaratus was discovered, but the Spartans allowed him to return when he began gathering an army in the surrounding territories. However, according to Herodotus he was by this time insane, and the Spartans put him in prison. He then tried to escape by cutting himself into pieces, and died as a result.

He was succeeded by Leonidas I, who married his daughter Gorgo.