Lake Idro (, also Eridio from , , ) is an Italian prealpine lake of glacial origin situated largely within the Province of Brescia (Lombardy) and in part in Trentino.
At 368 m above sea level it is the highest of the Lombard prealpine lakes. The lake is fed principally by the waters of the river Chiese; that river is also its only emissary. It has a surface area of 11.4 km<sup>2</sup> and a maximum depth of 122 m.
The lake is surrounded by wooded mountains. The shoreline of some 24 km is shared across four communes: Idro (the frazioni Crone and Lemprato), from which the lake takes its name, Anfo, Bagolino (fraz. Ponte Caffaro) and Bondone (fraz. Baitoni).
Lake Idro currently has become a site of conflict between environmental, agricultural and electricity industry interests.
The name derives from a legendary monster (Idra) who supposedly lived there.