Sint-Gertrudiskathedraal (Saint Gertrude's Cathedral) serves as the main church of the Old Catholic Church and the seat of the Old Catholic Archbishop in the Netherlands. The church was built between 1912 and 1914 in the neo-Romanesque style, chosen as a tribute to the long-lost Saint Mary's Church that once stood nearby.

The cathedral's towers dominate its front side, while a three-sided apse completes the structure. Inside the murals, primarily geometric and abstract, add to the cathedral’s unique atmosphere. Its inventory of religious artifacts, including paintings, sculptures, church silver, and relics, comes from various disused Old Catholic churches.