This work was designed in order to gain an insight on the mechanisms by which antioxidants prevent pancreatic disorders. We have examined the properties of cinnamtannin B-1, which belongs to the class of polyphenols, against the effect of hydrogen peroxide (H2O2) in mouse pancreatic acinar cells. We have studied Ca2+ mobilization, oxidative state, amylase secretion, and cell viability of cells treated with cinnamtannin B-1 in the presence of various concentrations of H2O2. We found that H2O2 (0.1–100 μM) increased CM-H2DCFDA-derived fluorescence, reflecting an increase in oxidation. Cinnamtannin B-1 (10 μM) reduced H2O2-induced oxidation of CM-H2DCFDA. CCK-8 induced oxidation of CM-H2DCFDA in a similar way to low micromolar concentrations of H2O2, and cinnamtannin B-1 reduced the oxidant effect of CCK-8. In addition, H2O2 induced a slow and progressive increase in intracellular free Ca2+ concentration ([Ca2+]c). Cinnamtannin B-1 reduced the effect of H2O2 on [Ca2+]c, but only at the lower concentrations of the oxidant. H2O2 inhibited amylase secretion in response to cholecystokinin, and cinnamtannin B-1 reduced the inhibitory action of H2O2 on enzyme secretion. Finally, H2O2 reduced cell viability, and the antioxidant protected acinar cells against H2O2. In conclusion, the beneficial effects of cinnamtannin B-1 appear to be mediated by reducing the intracellular Ca2+ overload and intracellular accumulation of digestive enzymes evoked by ROS, which is a common pathological precursor that mediates pancreatitis. Our results support the beneficial effect of natural antioxidants in the therapy against oxidative stress-derived deleterious effects on cellular physiology.