The word Poros means a strip of sea, narrower than a strait, uniting two little bays. During ancient times, according to the author Pausanias, you could walk across it!

In the North-Eastern part of the island, archaeologists discovered the ruins of a settlement from the Early Bronze Period which is believed to be interrelated with the wreck found on the nearby Dokos island.

In 1484 the Venetians occupied Poros and used it as a strategic port in their sea battles. Poros was the most powerful city in the region, and during that time the island had about 15,000 inhabitants, making it one of the largest cities in Greece. 

Poros had an important role during the Greek Revolution in 1821, due to its strategic position. 

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. As Russian naval activity grew, land was acquired at the edge of Poros town. Extensive material, coal, and food storage facilities were built, as well as a hardtack baking factory. 

After Greek independence, Governor Capodistrias requisitioned the facilities for use of the Greek Navy, and offered the Russians an alternative location in a nearby cove. The new facilities were far more extensive and were used by Russian ships throughout the 19th century. The number of Russian residents of Poros increased and a Russian school was even established. Then as Russian naval activity declined, so did the base and by the early 20th century only a single Russian watchman was left guarding it. It was then granted to the Greek navy by the Czar but was never commissioned, and the abandoned buildings were left to decay. The ruins, in elaborately carved stone, were listed as protected architectural monuments in 1989.