The historic centre of Siena has been declared by UNESCO a World Heritage Site.

Siena's cathedral, the Duomo, begun in the twelfth century, is one of the great examples of Italian romanesque architecture. Its main façade was completed in 1380. It is unusual for a Christian cathedral in that its axis runs north-south. This is because it was originally intended to be the largest cathedral in existence, with a north-south transept and an east-west aisle, as is usual. 

Inside is the famous Gothic octagonal pulpit by Nicola Pisano (1266–1268) supported on lions, and the labyrinth inlaid in the flooring, traversed by penitents on their knees. Within the Sacristy are some perfectly preserved renaissance frescos by Ghirlandaio, and, beneath the Duomo, in the baptistry is the baptismal font with bas-reliefs by Donatello, Ghiberti, Jacopo della Quercia and other 15th century sculptors. The Museo dell'Opera del Duomo contains Duccio's famous Maestà (1308–1311) and various other works by Sienese masters. More Sienese paintings are to be found in the Pinacoteca.

The shell-shaped Piazza del Campo, the town square, which houses the Palazzo Pubblico and the Torre del Mangia, is another architectural treasure, and is famous for hosting the Palio horse race. The Palazzo Pubblico, itself a great work of architecture, houses yet another important art museum. Included within the museum is Ambrogio Lorenzetti's series of frescos on the good government and the results of good and bad government and also some of the finest frescoes of Simone Martini and Pietro Lorenzetti.

On the Piazza Salimbeni is the Palazzo Salimbeni, a notable building and also the medieval headquarters of Monte dei Paschi di Siena, one of the oldest banks in continuous existence and a major player in the Sienese economy.

Housed in the notable Gothic Palazzo Chigi on Via di Città is the Accademia Musicale Chigiana, Siena's conservatory of music.