The history of Gythio dates from the prehistoric times, as many archaeological findings confirm. According to ancient Greek mythology, Heracles and Apollo were the founders of Gythion, During antiquity, it was the main port of Sparta. In 455 BC, it was burned by Tolmides, the Athenian admiral, during the First Peloponnesian War but it overcame that and was rebuilt. During the Roman-Spartan War in 195 BC, Gythio was seized and captured with the support of a combined Roman, Pergamese, and Rhodian fleet. At the end of the war, the city of Gythio became the founder of the Union of Free Laconians under Achaean protection and was finally liberated by a Roman fleet under the command of Atilius.
In the following years, between 195 BC-297 AC, Gythio became an independent town and capital of the Koinon of Free Laconians also called the "Eleutherolaconian" towns, which were a community of twenty-four cities, formed to maintain their autonomy against Sparta and was eventually declared free by Caesar Augustus. The town was embellished with several marbles, temples, sanctuaries and many works of art. Remains from that time survive to this day in the ancient town of Gythio and the ancient theatre.
Its prosperity lasted for a while, in 375 BC there a was a catastrophic earthquake and the tidal wave sunk the entire town and its inhabitants, those who could not make it to the hill. Most of the remains, ancient marbles and statues were found in the depths of the sea. As time passed, the fortifications buried the last traces of ancient remains. During medieval times, Gythio was completely abandoned. It was named Marathonisi from the known plant, marathos. In the 17th century, this area was full of marshes and wildings and with no human traces. The inhabitants of Mani avoided going to Gythio due to the fatal diseases that were spread in the area like malaria and the pirate raids.
In 1685, Venetians fought against the Turks and with the help of Maniates, they occupied the castle of Passava and killed the guards. The occupation of this castle has been a great event in the history of Mani, giving locals the freedom of communicating with Marathonisi. So, at a very slow pace, after two years, the legendary admiral Frangiskos Morozinis revived the town of Gythio or else Marathonisi. In the milestones of its modern history, perhaps the most important event is the opening of a port, which started functioning in 1960. Today, Gythio is the capital of Mani and the second largest city in Laconia.