Sant'Agata Fossili is a comune (municipality) in the Province of Alessandria in the Italian region Piedmont, located about southeast of Turin and about southeast of Alessandria. As of 31 December 2004, it had a population of 425 and an area of .
Sant'Agata Fossili borders the following municipalities: Carezzano, Cassano Spinola, Castellania Coppi, Sardigliano.