largest lake of the Italian peninsula in Umbria region, central Italy, 10 miles (16 km) west of Perugia. It has an area of 49 square miles (128 square km) and is shallow, its maximum depth being 20 feet (6 m). The lake it is fed by small streams and has an artificial subterranean outlet (opened 1898) to the Tiber River. Surrounded by hills on three sides, with an open lowland to the west, Trasimeno is often subject to heavy storms created by violent winds, especially from the north and west. There are three islets in the lake: Maggiore, Minore, and Polvese. The lake's shores, formerly marshy and malarial, are sparsely inhabited, with only two villages, both popular bathing resorts, Castiglione del Lago and Passignano sul Trasimeno.

The lake is known in Roman history as the scene in 217 BC of the bloody defeat by Hannibal of a Roman army under the consul Gaius Flaminius (Flaminius, Gaius). At a site on the north shore the precise location of which is still controversial, Hannibal's African, Iberian, and Celtic troops ambushed the Roman army in a narrow defile in the hills close to the lake, killing about 15,000 of the Romans, including the consul, and capturing about 6,000.