Poros, called Calaureia in ancient times, was the island of Poseidon, god of the sea. This is evident from various literary references and from the traces of the temple of Poseidon, which are to be found in the middle of the island. This fact may account for why Poros was chosen to be the headquarters of an amphictyony (alliance) set up in the 7th century BC whose members were seven cities.

Subsequently, the fortunes of Poros followed those of the other islands of the Saronic Gulf. In the War of Independence against the Turks in 1821, Poros fought alongside the other islands, and in 1830 became Greece's first naval base.