Spartan king (235–222) who reorganized Sparta's political structure and struggled unsuccessfully to destroy the Achaean League. A member of the Agiad house, he was the son of King Leonidas II. The conflict with the Achaean League under Aratus Of Sicyon began in 229. In 227 Cleomenes defeated the Achaeans at Mt. Lycaeum and at Ladoceia near Megalopolis. The next year he captured Mantineia and severely defeated the Achaeans at Hecatombaeum, near Dyme. After Cleomenes took Pellene, Phlius, Argos, and other cities, Aratus was forced to call upon King Antigonus Doson (Antigonus III Doson) of Macedonia for assistance. Antigonus failed to pierce Cleomenes' lines near Corinth in 224, but a revolt against Cleomenes at Argos put the Spartans on the defensive. Finally, in 222, Antigonus defeated Cleomenes at Sellasia (north of Sparta). Sparta fell to the Macedonian king, and Cleomenes fled to King Ptolemy Euergetes in Egypt. Imprisoned by Euergetes' successor, Ptolemy Philopator, Cleomenes escaped in 219 and, after failing to raise a revolt in Alexandria, took his own life.

The reforms imposed by Cleomenes in 227 were somewhat similar to those attempted earlier by the Spartan king Agis IV (died 241 BC). Cleomenes cancelled debts, redivided the land to provide 4,000 new citizen holdings, and restored the old Spartan training of youth. The Ephorate (ephor), five elected magistrates who, with the king, formed the main executive body of the state, was abolished (four of the five ephors being executed); the powers of the council were probably curtailed; and patronomoi (the board of six elders) was probably introduced at this time. In addition the army was trained to use a longer pike. Cleomenes' system was designed to re-create a society of aristocrats while neglecting helots (serfs) and perioikoi (a special class of disenfranchised inhabitants).