Kos is the birthplace of Hippokrates, father of medical science. It is near Halicarnassus of Mikra Asia which is the birthplace of Herodotus, father of history science.

In the 14th century B.C., Achaeans settled on the island. According to Homer, Kos participated in the Trojan War. In the 7th century B.C., Kos, along with three cities of Rhodes and two of Minor Asia, joined their forces and founded the alliance of the Doric "Exapolis". In the 5th century, kos was conquered by Persians, but after the defeat in Salamina, in 479 B.C. it was liberated by Athenians. In 357 B.C., after Hippokrates' death, it was built a hospital that became known all over the Mediterranean.

During Byzantine period, the island had great development, especially in the production of textiles, despite the pillages of Saracenes. In 1315, the Knights of Saint John conquered th island, until 1523, when Turks invaded and occupied it. In 1948, Kos became part of the Greek State, after Italians withdrew from Dodecanesse.