The castle of Rio is situated in Rio and the region of Achaea.

The Rio Castle is next to the Rio-Antirrio Bridge and is surrounded by sea, except from the side facing towards the land, which is protected by a moat with seawater. On the side of the castle lies the pier where ships that take the route Rio-Antirrio can dock.

The Rio Castle was built in 1499 by Sultan Bayezid. In the castle's location,there were ruins of the temple of Poseidon. The construction of the castle lasted only three months and together with another one located opposite in Antirrio, they offered absolute control of the sea passage. In 1687 the Venetian Francesco Morosini occupied the fort after a siege. The damage caused by the battle was so great that he was virtually forced to rebuild it, in the form it has today. In 1715 the Castle of Rio is re-conquered by the Turks, and in 1828 after a siege of the French general Maison, it is finally delivered to the Greeks. The castle was used for a long time as a prison, while during the Second World War a German guard was installed there.

Today the Castle of Rio is open for visits and is occasionally used for cultural events.