Demaratus () was a co-king of Sparta from 515 until 491 BC. He succeeded his father Ariston as king. In 491 Demaratus and his co-king Cleomenes I became bitter rivals. Cleomenes plotted to have his kinsman Leotychides replace Demaratus as king. Cleomenes bribed the Oracle of Delphi to say that Demaratus was not actually the son of Ariston. Demaratus lost his kingship. But he still held a position as a magistrate in Sparta. After being ridiculed by the king over no longer being a king himself, Demaratus went into exile. He went to Darius I of Persia who gave him cities to rule over. Demaratus later went with king Xerxes I on his invasion of Greece in 480 BC. He warned Xerxes not to underestimate the Spartans before the Battle of Thermopylae.