Castellar Guidobono is a comune (municipality) in the Province of Alessandria in the Italian region Piedmont, located about east of Turin and about east of Alessandria.
Castellar Guidobono borders the following municipalities: Casalnoceto, Viguzzolo, and Volpeglino.
Linked to the municipality of Tortona, whose destiny it followed, it was a fief of the Guidobono Cavalchini family of Monleale, from which it derives its name.