Occupied since prehistoric times, the canton of Valais saw the rise of an exceptional civilization during the Bronze Age. From the 4th century BC, four Celtic tribes shared its territory, which was incorporated into the Roman Empire by Augustus. The Gallo-Roman Valais, located on the important Great St Bernard Pass, was prosperous. Christianity was first established in 377, and a bishopric was opened in Martigny by 381 at the latest.
With the fall of the Empire, the region became Burgundian, before being integrated into the Carolingian Frankish kingdom. On its demise, it became part of the Transjurane kingdom of Burgundy, of which the abbey of Saint-Maurice d'Agaune was the religious center. The county of Valais became the property of the bishop in 999, upon donation by Rudolph III of Burgundy. In the 11th century, it passed to the Holy Roman Empire, and feudalism fragmented it into numerous seigneuries and territories. The upper part of the territory became Germanized in successive waves between the 9th and 14th centuries, while the influence of the Counts, then Dukes, of Savoy increased in the Lower Valais. At the end of the 14th century, it became an ally of the Swiss cantons, and its border was fixed at the Morge de Conthey. During the Burgundian Wars, the Upper Valais invaded Savoy territories, annexing the Lower Valais as far as Massongex and organizing it as a subject country. In 1569, Chablais Valais was conquered, again to the detriment of Savoy. The Lower Valais remained subject to the Upper Valais, which saw the bishop's power diminish in favor of the Patriotes, who in 1634 formed a genuine federal republic, the Republic of the Seven Tithings.
It wasn't until the French Revolution that the Lower Valais gained its independence. Valais fluctuated between the Helvetic Republic (1798-1802), theoretical independence (1802-1810) and incorporation into the Empire (1810-1813). Following the fall of Napoleon I, the Allies persuaded Helvetia to join the Swiss Confederation in 1815, making it the twentieth canton.
Still torn between the German-speaking upper and French-speaking lower cantons, it came close to splitting into half-cantons (1840). As a member of the Sonderbund (1845-1847), it was defeated. The second half of the 19th century saw the development of transport (rail and road) and the beginnings of tourism, while the early 20th century saw the industrialization of the country (chemicals in Monthey and Visp, aluminum in Chippis) and the exploitation of water resources. From 1950 onwards, mass tourism developed and numerous resorts sprang up.