Following the island's topography and geology, small creeks and seasonal streams flow through steep valleys of the southern and northeastern part.

[5] In the northeastern part of the island, in a location called “Kavos Vasili”, the archaeologists have discovered the ruins of a settlement of the Early Bronze Age.

This settlement is the oldest of the wider area of Trizinia, Peloponnese, and is believed to be interrelated with the wreck found on the nearby Dokos island which dates to the same period.

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.

[citation needed] 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.

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.

[citation needed] An enormous feast was found dating to the Hellenistic period in the ruins of the Calaurian asylum, along with a plaque celebrating the "revival" of its amphictyony.

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.

Poros was the most powerful city of the wider area, also governing Methana Island, Epidaurus, Damalas (Trizina), Fanari and Valario.

In September 1828, the ambassadors of England, France and Russia met in Poros with Ioannis Kapodistrias in order to determine the borders of the future Greek state, which was established two years later, in 1830.

With the Treaty of Kuchuk Kainarji, Russia secured free shipping for its navy, war and merchant alike, throughout the waters of the Ottoman Empire.

As Russian naval activity grew, need arose for a supply station, and land was acquired at the edge of Poros town.

In the beginning of the 20th century, among the activities of the Poros' inhabitants were agriculture (mainly wheat, grapevines and olives), livestock, fishing and shipping.