They were conquered by the Romans in the second decade BC. By 8-7 BC, Emperor Augustus praised the tribe (civitas) of Seduner with an inscription. The town-hall is said to contain several Roman inscriptions, one of which found at Sion commemorates the Roman presence: Civitas Sedunorum Patrono. Under the Romans it was known as Sedunum.
The Roman settlement stretched mainly from what is now St. Theodul, between the Sionne and to the west side of the hill, Valeria. Under the church, a large bath complex was discovered and partially excavated. Near La Sitterie, Sous-le-Scex and in the upper part of the Avenue du Petit Chasseur, portions of several villae suburbana were found. In the first Century AD, the Claudii Vallensium Forum, in what is now Martigny, became the capital of the civitas Vallensium. Sedunum lost political importance, but still remained the home of many notable families. Grave stelae attest to the presence of duumviri (magistrates of the civitas), of flamines (priest of the imperial cult), a Roman knight and a former consuls in the town. In the 4th Century Praesides (provincial governors) are mentioned living in Sedunum, including a man named Pontius Asclepiodotus, who rebuilt an imperial building and, according to an inscription, converted to Christianity in 377.
Seat of a catholic bishop
The Roman Catholic diocese of Sion is the oldest in Switzerland and one of the oldest north of the Alps. At first the see was sited at Octodurum, now called Martigny/Martinach. The first authentically historical bishop was Saint Theodore/Theodolus (died in 391), who was present at the Council of Aquileia in 381. He founded the Abbey of Saint-Maurice, with a small church in honor of Saint Maurice, martyred there ca. 300, when he united the local hermits in a common life, thus beginning the Abbey of Saint-Maurice, the oldest north of the Alps. Theodore rebuilt the church at Sion, which had been destroyed by Emperor Maximinus at the beginning
