Poros (Greek: Πόρος) is a small Greek island-pair in the southern part of the Saronic Gulf, about 58 km south from the port of Piraeus and separated from the Peloponnese by a 200m wide sea channel, with the town of Galatas on the mainland across the strait.
Poros consists of two islands: Sphaeria (Greek: Σφαιρία), the southern part, which is of volcanic origin, where today's city is located, and Kalaureia (Greek: Καλαυρία), also Kalavria or Calauria (meaning 'gentle breeze'), the northern and largest part. A bridge connects the two islands over a narrow strait.
Poros was divided into two islands during antiquity: Sphairia and Calauria. Sphairia consisted of the area of the modern island which includes its current capital. Calauria was the larger part of the island to its north. During the period of Mycenaean dominance (1400-1100 BC) Calauria was quite powerful and the most important naval base of region was located on islet Monti or Liontari on its eastern coast. The city-state of Calauria was home to an asylum dedicated to Poseidon, the ruins of which are still accessible on a hilltop close to the town. This asylum may have been linked to the sanctuaries at Geraistos and Tainaros. Ancient historians stated that Calauria was the center of an amphictyony, a religious alliance between its city-state and those of Athens, Poros, Aegina, Epidaurus, Hermione, Troezen, Nauplio, Orchomenus, and Prasaiai.
During the 5th century, the Persian Empire annexed the Greek cities of western Anatolia. When the Athenians helped these cities in a revolt against Persian rule, Poros, along with the other Greek states of mainland Greece and the Aegean islands, was drawn into the Greco-Persian Wars. At the beginning of the Peloponnesian War, Troezen and Calauria offered asylum to an anti-Macedonian politician who eventually became the tyrant of the region. After the death of Alexander the Great in 323 BC, the Ptolemies of Egypt occupied Calauria.
In 273 BC, the last explosion of the Methana volcano dramatically changed the morphology of Poros and the wider region.