It was first settled as an agricultural suburb of Carsulae in the 1st century along the Via Flaminia. This ancient settlement may have been called Casventium. During the Middle Ages, as Carsulae was abandoned, Casventum grew to become a small city and, eventually, a fortified border outpost. Its name was changed to San Gemini and there are various theories as to why this happened. The official story is that a Syrian monk named Gemine (Yamin) stopped to preach there, and then made his home in the town, thus inspiring the name change. Another theory is that it derives from a local popular cult of the twin saints Cosma and Damian.
San Gemini has been inhabited, without interruption, since ancient Roman times. It reached maximum expansion in pre-industrial times (the 13th century) and then stagnated until the 20th century. The city started to grow again in the 1930’s and has now reached a population of five thousand. Since the Second World War, the city has become a bedroom community for the city of Terni. Most of the growth that has happened since 1960 is outside the medieval perimeter. Although the population of the historic center has decreased by nearly ten percent, it still remains the civic and social center of the city.
Across Italy, San Gemini is mostly known for “Sangemini” natural mineral water. Both the spring and the bottling plant are located outside the populated center, near the ancient city of Carsulae.