Demaratus (Greek: Δημάρατος) was a king of Sparta from 515 until 491 BC, of the Eurypontid line, successor to his father Ariston. As king, he is known chiefly for his opposition to the other, co-ruling Spartan king, Cleomenes I.
When Cleomenes attempted to make Isagoras tyrant in Athens, Demaratus tried unsuccessfully to frustrate his plans. In 491 BC, Aegina was one of the states which gave the symbols of submission (earth and water) to Persia. Athens at once appealed to Sparta to punish this act of medism, and Cleomenes I crossed over to the island to arrest those responsible. His first attempt was unsuccessful, due to interference from Demaratus, who did his utmost to bring Cleomenes into disfavour at home.
In retaliation, Cleomenes urged Leotychidas, a relative and personal enemy of Demaratus, to claim the throne on the ground that the latter was not really the son of Ariston, but of Agetus, his mother's first husband. Cleomenes bribed the Delphic oracle, to pronounce in favor of Leotychidas, who became king in 491 BC.
After the deposition of Demaratus, Cleomenes visited the island of Aegina for a second time, accompanied by his new colleague Leotychides, seized ten of the leading citizens and deposited them at Athens as hostages.
On his abdication, Demaratus was forced to flee. He went to the court of the Persian king Darius I, who gave him the cities of Pergamum, Teuthrania and Halisarna, where his descendants Eurysthenes and Procles still ruled at the beginning of the 4th century.
|“||The same goes for the Spartans. One-against-one, they are as good as anyone in the world. But when they fight in a body, they are the best of all. For though they are free men, they are not entirely free. They accept Law as their master. And they respect this master more than your subjects respect you. Whatever he commands, they do. And his command never changes: It forbids them to flee in battle, whatever the number of their foes. He requires them to stand firm -- to conquer or die. O king, if I seem to speak foolishly, I am content from this time forward to remain silent. I only spoke now because you commanded me to. I do hope that everything turns out according to your wishes.||”|
— Herodotus vii (trans. G. Rawlinson)
Xerxes I also asks Demaratus about his knowledge of the Greeks and if they will put up a fight against the Persian army. Demaratus defends the Greeks even after being deposed and exiled from Sparta: 
|“||So Demaratus said, 'my lord, you have asked me to tell the whole truth—the kind of truth that you will not be able to prove false at a later date. There has never been a time when poverty was not a factor in the rearing of the Greeks, but their courage has been acquired as a result of intelligence and the force of law. Greece has relied on this courage to keep poverty and despotism at bay. I admire all the Greeks who live in those Dorian lands, but I shall restrict what I have to say to the Lacedaemonians alone. First, then, there's no way in which they will ever listen to any proposals of yours which will bring slavery on Greece; second, they will certainly resist you, even if all the other Greeks come over to your side. As for the size of their army, there's no point in your asking how, in terms, of numbers, they can do this. if there are in fact only a thousand men to march out against you (though it may be fewer or it may be more), then a thousand men will fight you.'||”|
— Herodotus vii (trans. Robin Waterfield)
Later King or Tyrant Nabis claimed to be of his descent.
- Xenophon Anabasis, ii. j. 3, vii. 8. 17; Hellenica, iii. I. 6
- Athenaeus i. 29 f
- Herodotus v. 75, vi. 50-70, vii ;
- Pausanias iii. 4, 3-5, 7, 7-8;
- Diodorus xi. 6;
- Polyaenus ii. 20;
- Seneca the Younger, De benefici-is, Vi. 31, 4-12
|Eurypontid King of Sparta
C. 515 BC – c. 491 BC
- Herodotus (1998). The Histories. New York: Oxford University Press.