The octagonal tower of Castellone
Formia was founded in ancient times by the Laconi and named in Greek, Ὁρμίαι (hormiai, meaning "landing place") and later in early Latin, Ormiae. In the Roman Republic era it was called Formiae (derived from Hormia or Ormiai, for its excellent landing). It was a renowned resort during the imperial era.
Cicero was assassinated on the Appian Way outside the town in 43 BC, and his tomb remains a minor tourist destination. The city was also the seat of St. Erasmus's martyrdom, by being disemboweled around 303 AD, during the persecutions of Diocletian. St. Erasmus later also became known as Saint Elmo the patron saint of sailors.
After the fall of the Western Roman Empire, the city was sacked by barbarians and the population moved to two distinct burghs on the nearby hill, which were under the rule of Gaeta. Charles II of Anjou built a fortress in the maritime burgh, Mola di Gaeta. The other burgh was known as Castellone, from the castle erected there in the mid-14th century by Onorato I Caetani, count of Fondi.
The two villages were united again in 1863 under the name of Formia. The reunited city was badly damaged in 1943–44 in bombing operations and the Battle of Anzio.