Chiasso (Italian pronunciation: [ˈkjasso]; Lombard: Ciass [ˈtʃas]) is a municipality in the district of Mendrisio in the canton of Ticino in Switzerland .

As the southernmost of Switzerland's municipalities, Chiasso is on the border with Italy, in front of Ponte Chiasso (a frazione of Como , Italy). The municipality of Chiasso includes the villages of Boffalora, Pedrinate and Seseglio.

In 2007, the three mayors of Chiasso, Vacallo and Morbio Inferiore decided to unite into one commune. The new united commune with a population of ~15,300 people over a territory of 9.2 km2 (3.6 sq mi), was rejected by the population in November 2007.

Chiasso is first mentioned in 1140 as Claso.