The site began to be mentioned in historic sources from the XI century, when the inhabitants of the ancient Sulma moved to the place where Sermoneta is found today to escape from the raids of the Saracens and the expansion of the Pontine Marches.
However, the real town has medieval origins. Already under the control of the powerful Caetani family from 1297, Sermoneta became the centre of their rule over the whole of southern Lazio.
In fact the town enjoyed a strategic position along the Via Pedemontana, the hill road which at the time replaced the Appia in links between North and South Italy.
The medieval borgo dates back to the period of control of the Caetani and still splendidly preserves the Caetani Castle and the Duomo.
With the first works to claim the Agro Pontino and the subsequent reopening of the Appia since the XVI century, Sermoneta started a slow decline and in 1597 lost the title of capital of the Duchy of the Caetani which moved its central seat to Cisterna.
Today the attractive borgo is a popular tourist destination and the venue of numerous important cultural events.