San Costanzo is a comune (municipality) in the Province of Pesaro e Urbino in the Italian region Marche, located about 40 kilometres (25mi) northwest of Ancona and about 20 kilometres (12mi) southeast of Pesaro. As of 31 December 2004, it had a population of 4,339 and an area of 40.7 square kilometres (15.7sqmi).