Above the roofs of the half-timbered houses in the Altstadt rise the six towers of St. Martin's Cathedral, the most important Catholic cathedral in the country after Cologne's. It dates from A.D. 975 but was continually rebuilt and restored, reaching its present form mainly in the 13th and 14th centuries. Below the largest dome, a combination of Romanesque and baroque styles, is the transept, separating the west chancel from the nave and smaller east chancel. Many of the supporting pillars along the aisles of the nave are decorated with carved and painted statues of French and German saints. Among other impressive furnishings in the sanctuary are rococo choilr stalls and an early 14th century pewter baptismal font.

The cathedral's Diocesan Museum houses a collection of religious art. In it are exhibitions of reliquaries and medieval sculpture, including works by the Master of Naumburg, the anonymous 13th century artist who sculpted many of the statues in cathedrals across Northern France and Germany. In the 1,000-year-old cathedral crypt is a contemporary gold reliquary of the saints of Mainz.