The Arno is a river in region of Tuscany, Italy, that crosses all the region from Mount Falterona (near the city of Arezzo, in Casentino area), to Pisa where it enters Tyrrhenian Sea. It is the biggest river in the region.

It crosses Florence too, where it passes below the Ponte Vecchio. The river flooded the city regularly in historical times, the last occasion being the famous flood of 1966.

While classified as a river, the Arno is sometimes described as having a torrent-like behaviour, because it can easily go from almost dry to near-flood in a few days. New dams built upstream of Florence have greatly alleviated the problem in the last years.