ancient Etruscan town on the site of modern Chiusi, in Tuscany regione, north-central Italy.

Clusium was founded in the 8th century BC on the site of an older Umbrian town known as Camars. In the early 6th century BC it entered into an alliance with Arretium (Arezzo) and other Etruscan towns against Tarquinius Priscus of Rome. At the end of the 6th century BC, Clusium's king, Lars Porsena, attacked Rome and may even have captured the city in an attempt to restore the power of the Tarquins there. In 391 BC Clusium was allied with Rome against invading Gauls led by the chieftain Brennus, who was able to capture Rome the following year.

Like other Etruscan cities, Clusium was surrounded by cemeteries and tombs. The modern excavation of Clusian tombs has yielded earthenware funerary jars with lids molded in the form of human heads, as well as ceramic human figures and Greek and locally made pottery. The decorations on sarcophagi found in the tombs have proved a major source of inscriptions in the Etruscan language. The modern town of Chiusi is a market centre with a growing tourist industry based on its Etruscan remains.