Leonidas I was king of the Ancient Greek city-state of Sparta. He was the son of king Anaxandridas II and the 17th king of the Agiad dynasty, a Spartan royal house which claimed descent from the mythical demigod Heracles. Leonidas I ascended to the throne in c. 489 BC, succeeding his half-brother king Cleomenes I. He ruled jointly along with king Leotychidas until his death in 480 BC, when he was succeeded by his son, Pleistarchus.