The city was probably founded in the 7th century BC by the Sabini. In the 3rd century BC it was conquered by the Romans and soon become an important municipium lying on the Via Flaminia. The Roman name was Interamna, meaning "in between two rivers". During the Roman Empire the city was enriched with several buildings, including aqueducts, walls, amphitheaters, temples and bridges.
After the Lombard conquest (755) Terni lost any role of prominence, reducing to a secondary town in the Duchy of Spoleto. In 1174 it was again destroyed by Frederick Barbarossa's general, Archbishop Christian of Mainz. In the following century Terni was one of the favourite seat of St. Francis' prayings.
In the 14th century Terni issued a constitution of its own and from 1353 the walls were enlarged, and new channels were opened. As well as much of the Italian communes of the Late Middle Ages, it was slained by inner disputes between Guelphs and Ghibellines, and later between the two parties of Nobili and Banderari. Later it become part of the Papal States. In 1580 an ironwork, the Ferriera, was introduced to work the iron ore mined in Monteleone di Spoleto, starting the traditional industrial connotation of the city. In the 17th century, however, Terni declined further due to plagues and famines.
In the 19th century Terni took advantage of the Industrial Revolution and of the large presence of water sources in the area. New industries included a steelwork, a foundry, as well as weapons, jute and wool factories. In 1927 Terni became capital of province. The presence of a strong industries concentration made it a favourite target for the Allied bombardments in World War II, totalling 108 raids. Many quarters and public edifices were destroyed.