CHIUSI (anc. Clusium), a town of Tuscany, Italy, in the province of Siena, 55 m. S.E. by rail from the town of Siena, and 26 m. N.N.W. of Orvieto. Pop. (1901) 6011. It is situated on a hill 1305 ft. above sea-level, and is surrounded by medieval walls, in which, in places, fragments of the Etruscan wall are incorporated. The cathedral of S. Mustiola is a basilica with a nave and two aisles, with eighteen columns of different kinds of marble, from ancient buildings. It has been restored and decorated with frescoes in modern times. The campanile belongs to the 13th century. The place was devastated by malaria in the middle ages, and did not recover until the Chiana valley was drained in the 18th century. For the catacombs see Clusium.