The Cathedral of Santa Maria Assunta e San Giovanni Battista (Cathedral of the Assumption of Mary and Saint John the Baptist) is the most important and oldest religious building in Aosta. The church was founded at the end of the 4th century, when a Roman complex was converted into a Christian place of worship.

At the end of the 15th century and the beginning of the 16th century, a series of works on the body of the building gave it the appearance it still has today, such as the Renaissance-style façade built between 1522 and 1526, decorated with frescoes and terracotta sculptures.

The ïmpressive original structure of the cathedral was modified in the 11th century. The cathedral is rich in art and religious treasures: a precious cycle of 11th-century frescoes that decorate the vault was discovered in 1979, while the interior of the cathedral houses, among other things, a wooden crucifix from 1397, valuable artistic stained glass windows made between 1400 and 1500. The high choir, dominated by a crucifix of the fourteenth century, has thirty wooden stalls dating from 1469.

The building dominates the city thanks to its two bell towers, which with 60 meters in height are the tallest buildings in the entire Aosta Valley.