It is undoubtedly the most important church in the city after the Cathedral.
Its construction can be divided into four phases.
It was founded in 1077 thanks to donations for the erection of a sacred building. However, the first major transformation took place in 1454: thanks to the intervention of Bernardino de Lera, the building took on a Renaissance style; the ancient bell tower from this period is still preserved.
The greatest transformation, however, took place in the early decades of the 19th century with the neoclassical design by architect Luigi Voghera. The imposing façade is also attributed to this intervention. 
The works inside are important, above all the cycle of frescoes by Giulio Campo dedicated to the history of Sant’Agata.

The church housed the Sacra Tavola which according to tradition contained the relics of the saint. The Tavola is now kept in the Museo Diocesano.