Castelletto Monferrato () is a comune (municipality) in the Province of Alessandria in the Italian region Piedmont, located about east of Turin and about northwest of Alessandria. As of 31 December 2004, it had a population of 1,511 and an area of .
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.