Cleomenes I , Agiad king of Sparta (reigned c. 520–490 BC), son of Anaxandridas II by a second, bigamous union. His long, activist reign was one of the half-dozen most influential on record. He pursued an adventurous and at times unscrupulous foreign policy aimed at crushing Argos and extending Sparta's influence both inside and outside the Peloponnese. 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. It was during his reign, but not entirely according to his design, that the Peloponnesian League came formally into existence.