Antica basilica di San Giovanni Battista, Minor basilica in Monza, Italy

The ancient basilica of San Giovanni Battista in Monza no longer exists: it was demolished between the 13th and 14th centuries to make way for the present Duomo. Written sources describe a three-nave church preceded by a four-sided atrium, probably with a cruciform plan and three apses. Of the Lombard building, two marble slabs with graffito crosses are set into the Duomo's facade left of the prothyrum, and a tower over twenty metres high, used as a bell tower until 1606, stands east of the sacristy.

Founded around 595 by Queen Theodelinda as the palatine chapel of her royal palace in Monza, the basilica was already consecrated by 603, when Abbot Secondo of Non baptised Adaloaldo, her son. Theodelinda and Adaloaldo were buried there, in 628 and 626. Demolished between the 13th and 14th centuries together with the palace, it made way for the present Duomo.

The basilica was demolished between the 13th and 14th centuries; its site lies under the present Duomo. Monza, though in the Archdiocese of Milan, celebrates the Roman rite: the Aquileian rite used since Theodelinda's time was suppressed in 1596 by Patriarch Francesco Barbaro at the synod of Udine. Paul the Deacon records that Rodoald, who had opened his father Rotari's tomb, was struck at the throat whenever he tried to enter.

The basilica itself no longer exists: it was demolished between the 13th and 14th centuries and replaced by the present Duomo. What remains is inside and around the Duomo: three early medieval tombs under a glass panel in the left nave floor, two graffito marble slabs with votive crosses set into the facade left of the prothyrum, and tiles kept in the Museo del Duomo.

East of the present sacristy stands a tower over twenty metres high, the most substantial surviving remnant of the Lombard complex, perhaps part of Theodelinda's royal palace. It served as the bell tower until 1606. Two marble slabs with graffito votive crosses are set into the cathedral facade left of the prothyrum.