Sion, capital of Valais canton, southwestern Switzerland. It lies along the Rhône River, at the mouth of La Sionne River, southeast of Lake Geneva (Lac Léman). It originated as a Celtic and Roman settlement called Sedunum. Sion became the seat of a bishop in the late 6th century, and from 999 the bishops of Sion held the spiritual and temporal power in Valais, which they retained, at least in part, until 1798. The two hillocks that dominate the town are crowned by the remains of the 13th-century Château de Tourbillon and those of the Château de Valère (Valeria), now containing a historical museum and a mainly 13th-century church. Other notable buildings are the Cathedral of Notre-Dame-du-Glarier, the Church of Saint-Théodule (1516), the Maison Supersaxo (1505), the town hall (1660), and the remains of the Château de la Majorie, now housing an art gallery. The town is on the road and rail routes from Lausanne to Milan via the Simplon Pass. It is an important market for vegetables, fruit, and wine. The population is predominantly French speaking. Pop. (2007 est.) 28,633.