The construction of the Chiesa Maggiore (the Main Church - dedicated to Saint Peter the Apostle) of Guastalla began in May 1569, to a design commissioned by  Cesare Gonzaga from  Francesco Capriani da Volterra.

He chose to build the new church on the town's main square, modifying the city plan previously conceived by Domenico Giunti who, while giving Guastalla its central layout, had originally envisaged the church at the crossroads of the main road axis.

This new location profoundly reshaped the town’s structure, making the Chiesa Maggiore the defining element of the Gonzaga capital’s urban identity. The architectural model was that of a single nave with side chapels and a vaulted ceiling - a plan that would become the most common widespread during the Counter-Reformation.

On February 20th, 1575,  the first Sunday of Lent, Cardinal Carlo Borromeo - Cesare Gonzaga’s brother-in-law - consecrated the church, donating the relics of Saints Peter and Paul. Yet the occasion also marked Cesare’s funeral, as he died just three days before.

Completion works continued for several decades under the supervision of Pompeo Pedemonte, and the façade was finished only in 1619, crowned by the sculptural group of the Beata Vergine dei Popoli (Blessed Virgin of the People) placed in the central niche. In 1653, the opening of an arch between the central nave and the Oratorio di San Carlo (Oratory of Saint Charles) paved the way for the creation of the Cappella del Santissimo Sacramento (Chapel of the Blessed Sacrament), a prestigious Baroque addition designed by Antonio Vasconi. The dome was decorated with frescoes by Giovanni Battista Bolognini, a disciple of Guido Reni. These works were completed on July 31st, 1671.

The 18th century brought further transformation, giving the church much of its current appearance: in 1716, new bell towers were designed, and two years later the choir was demolished to enlarge the building under a project by engineer Abondio Bolla of Parma. On December 24th, 1784, the new high altar was consecrated: conceived by Giovan Battista Fattori, it was executed by stonemason Giovanni Rossi and embellished by gilder Angelo Bondenari.

In 1841, after its elevation to the rank of Co-Cathedral, the church underwent major works directed by Parma architect Rizzardi Polini, who redesigned the entablature, capitals, and arch frames, and created the stucco coffered ceilings of the dome and apse. At the same time, the floor was lowered and all tombs were removed, except for that of Duchess Teodora of Hesse-Darmstadt, widow of Duke Antonio Ferdinando Gonzaga.

The last systematic intervention was carried out in 1946 by engineer Alberto Paglia,  who built the marble staircase linking the nave to the presbytery. Since then, only minor adjustments have been made, until the comprehensive restoration works launched between 2011 and 2016.