Castelletto Monferrato is a comune (municipality) in the Province of Alessandria in the Italian region Piedmont, located about 70 km east of Turin and about 8 km northwest of Alessandria. As of 31 December 2004, it had a population of 1,511 and an area of 9.4 km2. The municipality of Castelletto Monferrato contains the frazioni (subdivisions, mainly villages and hamlets) Giardinetto and Gerlotti. Castelletto Monferrato borders the following municipalities: Alessandria, Quargnento, and San Salvatore Monferrato.