It was first settled as an agricultural suburb of Carsulae in the 1st Century along the Via Flaminia. This settlement was called Casventum. During the Middle Ages, as Carsulae was abandoned, Casventum developed into a small city. In the early Middle Ages its name was changed to San Gemini. There are various theories as to why this happened; the most popular being that a Syrian monk named Gemine stopped to preach and live there, inspiring the change, another is that it derives from a very ancient tradition of twin deities or saints associated to curative waters.