Castelletto d'Erro is a comune (municipality) in the Province of Alessandria in the Italian region Piedmont, located about 70 kilometres (43 mi) southeast of Turin and about 35 kilometres (22 mi) southwest of Alessandria.

Castelletto d'Erro borders the following municipalities: Bistagno, Cartosio, Melazzo, Montechiaro d'Acqui, and Ponti.

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