The patron of Santhià is Saint Agatha, celebrated on 5 February in the collegiate church dedicated to her. The building underwent various changes over the centuries until reaching its present form under architect Giuseppe Maria Talucchi between 1836 and 1839, and was consecrated by archbishop d'Angennes on 12 September 1841; the façade is neoclassical with a wide six-column pronaos, and the 12th-century Romanesque bell tower is among the oldest parts, together with the crypt of Saint Stephen, where tradition says the Lombard queen Theodolinda had the martyr's relics transferred. Inside, the church keeps the Polyptych of Saint Agatha by Gerolamo Giovenone.
Santhià is also home to the Carnevale Storico, Piedmont's oldest carnival, documented since at least 1328; since 2024 the Palazzo del Capitano has hosted the Carvè Museum dedicated to its history.
