Busto Arsizio (Italian: [ˈbusto arˈsittsjo] ; Bustocco: Büsti Grandi) is a comune (municipality) in the south-easternmost part of the province of Varese, in the Italian region of Lombardy, 35 kilometres (22 mi) north of Milan. The economy of Busto Arsizio is mainly based on industry and commerce. It is the fifth municipality in the region by population and the first in the province.

Despite some claims about a Celtic heritage, recent studies suggest that the "Bustocchi"'s ancestors were Ligurians, called "wild" by Pliny, "marauders and robbers" by Livy and "unshaven and hairy" by Pompeius Tragus. They were skilled ironworkers and much sought after as mercenary soldiers. A remote Ligurian influence is perceptible in the local dialect, Büstócu, slightly different from other Western Lombard varieties, according to a local expert and historian Luigi Giavini.