The Grand Som is a mountain of the Chartreuse Mountains in the French Prealps, in Isère, France. It has a long ridge, unevenly sharp, overlooking the Petit Som (1,772m), the Col de la Ruchère northwest and the Grande Chartreuse monastery below the cliffs. The summit has a cross and two relief tables. It lies in the Parc Naturel Régional de la Chartreuse.