Lake Pusiano, also called Casletto or Eupili (Lagh de Pusian in Lombard, Lacus Pusianum in Latin), is a lake located between the provinces of Como and Lecco, in Brianza, Lombardy.

In his honor Giuseppe Parini published his first book of poems (of arcane tone), under the pseudonym of Ripano Eupilino.

The painter Giovanni Segantini painted Lake Pusiano in his famous painting Ave Maria a trasbordo, which is among the first that mark the evolution towards pointillism.

Lake Pusiano is the largest of the Lakes Briantei, has a maximum length of 2,700 meters, a width of 2,400 meters, a perimeter of about 11 kilometers, a minimum area (in lean times) of 5,250,000 square meters that reaches 6,720,000 square meters in times of flood. The volume of water is 81 million cubic meters.

Lake Pusiano, like the other lakes of the Brianza area, was formed after the withdrawal of the huge glacier that covered much of the Brianza area and is therefore of glacial origin.

Initially, it formed a whole together with Lake Alserio, only in more recent times they separated due to the accumulation of sediment brought by the Lambro River and human activity of reclamation.

Its emissary is the Lambro, which is also the tributary with the name of Lambrone. Since 1811 the lake is regulated at the outlet to the valley of the Lambro through a dam, Cavo Diotti, now managed by the Regional Park of the Lambro Valley. On the banks of this stretch of water are the municipalities of Pusiano, Eupilio, Erba and Merone in the province of Como, Cesana Brianza, Bosisio Parini and Rogeno in the province of Lecco. Not far from Pusiano, in the territory entirely of Lecco there is another mirror of water a little more extensive called Lake Annone, from the town of Annone di Brianza.

In the lake there is the small Island of Cypresses, of private property. It is an oval-shaped islet that is located inside the lake of Pusiano. It owes its name to the presence of about 130 Cypresses, some of which are centuries old.