Trevi (Italian: [ˈtrɛːvi]; Latin: Trebiae) is an ancient town and comune (municipality) in Umbria, Italy, on the lower flank of Mount Serano [it] overlooking the wide plain of the Clitunno river system. Located in the Province of Perugia, it is 10 km (6 mi) SSE of Foligno and 20 km (12 mi) north of Spoleto. It is one of I Borghi più belli d'Italia ("The most beautiful villages of Italy").

The population of the comune was c. 8,000 in 2004, with the town proper accounting for about half of that; the rest lives in the frazioni (boroughs) of Borgo, Bovara, Cannaiola, Coste, Pigge, Manciano, Matigge, Parrano, Picciche, San Lorenzo and Santa Maria in Valle. The historical subdivisions of Trevi proper are the terzieri of Castello, Matiggia e Piano; they come into play only for the Palio.

Most of the town, densely inhabited and of decidedly medieval aspect, lies on sharply sloping terrain, only the very center being more or less flat. It commands one of the best views in Umbria, extending over 50 km (30 mi) in most westerly directions. Trevi is served by the main rail line from Rome to Ancona as well as the line from Florence to Rome via Perugia.

The territory of the comune extends over 71 km² from the valley (210 m) to Mts. Brunette (1422 m) and Serano (1429 m) and can be divided into three more or less equal zones: plain, hills, and mountain; each with its own characteristic vegetation. In the plain, the very fertile land is well irrigated by many watercourses, heirs for the most part to the Lacus Clitorius of Antiquity, and is suited to annual crops. The hills, of a very loose alkaline limestone with excellent drainage, are an ideal terrain for the intensive and highly specialized cultivation of olive trees that produce a very characteristic and highly prized oil. Finally, the mountains are covered in meadows and forests, primarily deciduous.

The natural environment of the territory as a whole remains largely unspoiled, this despite being significantly populated for millennia. The mountainous area is now witnessing a considerable depopulation — as elsewhere in Italy, in the second half of the 20th century settlements at the highest elevations have been rapidly losing their inhabitants to the plains. The most important growth areas since the period following World War II have been Borgo Trevi (commercial, residential, and offices), Pietrarossa (industrial), and Matigge (light and medium industry, commercial).

Many rivers and canals, none of them very large, run in a general south-to-north direction, converging into a single river near Bevagna. Their courses are by now highly artificial, the result of land and water management projects undertaken over many centuries, since they are recorded at least as early as the time of Theodoric the Great (6th century) and have continued down to our own time with the construction of dams and works to regularize the seasonal waters of the Marroggia that had been subject to frequent torrential overflows with sudden and disastrous results.

The most important of the year-round watercourses is the Clitunno River, celebrated in antiquity as the Clitumnus, whose deified waters were reputed to have miraculous properties and which have been lauded in prose and verse by Pliny the Younger, Propertius, Claudian, Addison, Byron and Carducci.

Trevi borders the following municipalities: Campello sul Clitunno, Castel Ritaldi, Foligno, Montefalco, Sellano, Spoleto.