Castelletto Merli is a comune (municipality) in the Province of Alessandria in the Italian region Piedmont, located about 40 kilometres (25 mi) east of Turin and about 35 kilometres (22 mi) northwest of Alessandria.

Castelletto Merli borders the following municipalities: Alfiano Natta, Cerrina Monferrato, Mombello Monferrato, Moncalvo, Odalengo Piccolo, and Ponzano Monferrato.

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