Casciago is a comune (municipality) in the Province of Varese in the Italian region Lombardy, located about 50 kilometres (31 mi) northwest of Milan and about 4 kilometres (2 mi) west of Varese. As of 1 December 2021, it had a population of 3.613 and an area of 4.0 square kilometres (1.5 sq mi).

Casciago borders the following municipalities: Barasso, Gavirate, Luvinate, Varese.

Casciago could very well be the location of the ancient Cassiciacum, where Saint Augustine and a few friends lived together in a kind of philosophical commune after Augustine's conversion to Christianity.