In Roman times, Pliny the Elder said Trevi was a city of the old Umbrians, and an old stone with Umbrian writing was found in the comune, at
Bovara, in the 1950s. "Treviae" is also listed in the 5th‑century
Bordeaux Itinerary. We do not know the history of Trevi in very old times, although some walls in the center part of the town on the hill are as old as the 1st century BC. Trevi started to spread out away from the hill during the time of the Empire, when Hadrian fixed the main road in the area, the
Via Flaminia; this made a small town in the plain grow, at a place now called Pietrarossa. For hundreds of years people have been finding old things there: there were Roman baths that people were probably using in the time of St. Francis, who came here and told people to bathe in them.
In old times people say that Trevi ruled the valley below it, all the way to the
Colli Martani, the line of mountains that run down the middle of Umbria. Trevi had a bishop until the 11th century, and was an important place belonging to the
Lombards (in Italian, a gastaldato). At the beginning of the 13th century, Trevi made itself independent and became a free commune. It often fought on the side of Perugia to defend itself against nearby Spoleto, and fought wars with other communes in the area, winning some and losing some. It was invaded by Spoleto in the 14th century and by the Trinci, rulers of Foligno. In 1438 Trevi became part of the lands belonging to the Church as part of the "legation" of Perugia: after that Trevi's history was as part of the States of the Church, then (1860) of the united Kingdom of Italy.
Trevi was at its most prosperous in the 15th century: the town was so important for those who wanted to buy and sell that people called it "il porto secco" — the dry port. In 1470, with Foligno, Trevi became the fourth town in Italy to have a printing press, managed by the first known printing company. Many big renaissance palazzi of the town indicate the contemporaneous prosperity.