The patron saint of Costacciaro is Blessed Thomas Grasselli, a Camaldolese hermit born in the town in 1262 and who died in 1337, celebrated on the first Sunday of September. His remains are kept in the church of San Francesco, dating from the 14th century, the town's most important monument, with a Gothic stone façade and a three-nave interior rich in works of art: a recent restoration uncovered several Templar symbols inside it. Some wall remains of the blessed's ancient dwelling still survive.
Costacciaro, of medieval layout with an orthogonal grid of streets of probable Roman origin, is also known for two distinct popular feasts: the Corsa delle botti, every second Sunday of November, in which two barrels pulled by young men called «bottaioli» race in competition between the main village and the hamlet of Villa Col de' Canali for the Trofeo di San Martino, and the Sagra dei Bigoli e dei prodotti del parco del Monte Cucco, on 17 and 18 August.
