Dedicated to the Assumption of the Virgin Mary, it is the seat of the Bishop of Acqui.

The ground plan is in the shape of a Latin cross, and there are five aisles (but until the 18th century, only three), terminating in three semi-circular apses.

The interior was re-fitted in Baroque woodwork, and was decorated with frescos and stucco work from the 17th to the 19th centuries.

The 19th century pulpit and the Baroque altar of Saint Guido in the transept are worthy of note.

The most important work of art in the cathedral by far however is the Retable of the Virgin of Montserrat, the work of the 15th century Spanish painter Bartolomé Bermejo, who painted the central panel around 1485, and by Rodrigo and Francisco de Osona who painted the side panels, in the chapter house.