The countryside surrounding Lake Trasimeno looks like a hilly amphitheater framing its surface (128 km2, the largest lake in peninsular Italy, and bound as a Regional Park). The western coast stretches into a flat strip open to the Val di Chiana. The northwest part of the Trasimeno territory marks the border area between the province of Perugia, of which the lake is integrally a part, with the neighboring provinces of Siena and Arezzo.

The origin of the lake basin is related to tectonic phenomena concomitant with the final stages of the Apennine orogeny. The lake is entirely within the Trasimeno District of the province of Perugia and lacks a natural emissary. The Trasimeno artificial outfall causes any excess water to flow into the Nestore River via the Caina stream. The Nestore in turn flows into the Tiber.

The lake is fed mainly by rainfall and by the Rigo Maggiore, Tresa, Moiano and Maranzano streams, which flow into the Anguillara artificial tributary. Paganico and Pescia are the two natural tributary streams of Trasimeno.