In mythology, Kea was known as Hydroussa because of its plentiful water and thick vegetation. Its woods were said to be inhabited by Nymphs, who abandoned the island, prompting Sirius, the brightest star in the sky, scorched Hydroussa and the other Cyclades and transform them into a barren landscape. The arid conditions and barren terrain forced the island's inhabitants to seek help from the demi-god Aristaeus of Thessaly, a son of Apollo and the Nymph Cyrene. The gods were appeased by the offerings made by Aristaeus and the waterlessness became less harsh. Since then, northerly winds known as meltemia blow over the islands for forty days when Sirius's constellation, the Big Dog, is visible.
Kea is named after the hero Keos, also a son of Apollo but with the Nymph Rhoedessa. Keos is believed to have lived on the island in the early eleventh century B.C. Finds from excavations at Kefalas suggest Kea has been inhabited since Neolithic times. Traces of prehistoric human habitation have been found on the Ayia Irini peninsula and at the village of Vourkari.
From the Bronze Age through the end of the Mycenaean period, the settlement at Ayia Irini made its mark on history and Early Aegean Civilization: in the sixteenth century B.C., Ayia Irini served as the hub between the Minoan and Mycenaean worlds, emerging as an important center for trade and culture.
The Ionians' arrival during the Archaic period led to the founding of the cities of Ioulis, Carthaia, Coryssus, and Poiessa, which also flourished, economically and culturally. Kea was famous for its political system, which caught the attention of Aristotle and is described in his "Keion Politeia".
The lawmaker Aristides, one of the seven sages of ancient Greece and known for his strict, model legislation, hailed from Kea. One of these laws was titled "Keion to Nomimon", according to which any citizen whose intellectual faculties and physical abilities were no longer beneficial to society should die. Thus, anyone over the age of seventy ended their own lives by drinking conium (or hemlock) from the mandrake. This practice ended in the third century A.D. as the population converted to Christianity.
From the dawn of the Byzantine Empire, Kea was included in the eparchy of Hellas and came under the administration of the Eastern Roman state. The independence of the island's four cities ended with the Roman empire, and Ioulis became Kea's sole center of culture.
Under Venetian rule, the acropolis of ancient Ioulis was fortified, while during the rule of the Justinian and Venetian dynasties (1207-1566), Kea was conquered successively by various Latin rulers.
It was during Venetian rule that the island became known as Tzia. In 1537, the island fell to the Turks; it subsequently suffered great destruction during the Russo-Turkish War.
Kea took an active part in the Greek war of independence, following the 1821 revolt, and officially became a part of the Modern Greek State in 1830.