The patron saint of Poggio Sannita is Saint Prosper the martyr, celebrated on 21 August. The town's church preserves the reliquary with an arm bone of the saint, brought in 1743 by Cardinal Vincenzo Petra, archbishop of Damascus and dean of the Apostolic Penitentiary, who had extracted it from the martyr's body in the catacomb of Calepodius in Rome, with the permission of Pope Clement XII. The same church also houses the historic 1769 organ, a masterpiece by Francesco D'Onofrio, a member of a Poggio Sannita family of organ builders whose works are found in many churches across Molise and the neighbouring regions.
The town also preserves the Palazzo Ducale, built in the 15th century as the residence of the dukes of Caccavone, the municipality's former name, and inhabited until the early 19th century by the Petra family, dukes of Vastogirardi and marquises of Caccavone. Next to the palace stands the church of Santa Vittoria, damaged by the 1720 earthquake and rebuilt in 1725 at the will of Duke Nicola Petra. Saint Prosper is venerated alongside other saints held dear in the town, including Saint Victoria, Saint Cataldus and Saint Anthony the Abbot.
