Porto San Giorgio was founded as a port of the nearby Fermo.
Plinio the Elder describes it as “Castellum Firmanorum”. In medieval documents it appears with the name of “Portus Firmi” (Harbour of Fermo) and it is indicated in the ancient nautical charts as seaport of primary importance.
Centre of the maritime traffic for the hinterland of Fermo to and from Venice, meant that, between these two cities was established an alliance. After the barbarian invasions and the subsequent destructions, in the area some fishermen settled and gave life to a town, which since 1164 was run by the Cathedral of Fermo.
In 1260 the castle was ceded to the Municipality of Fermo, who turned it into a fortress. Given the Turkish raids, especially in the Adriatic sea, Fermo decided to enclose all the village within the castle walls. In 1741 Porto San Giorgio was considered a separate Castle from Fermo and in 1782 the Papal Government granted to Porto San Giorgio the possession of the territories ranging from Tenna to Ete rivers.