Phenolic composition and antioxidant activities of the aqueous extract of Arisaema cum Bile, which is widely used as a folk medicine in Korea, were determined. Phenolic composition profile revealed that the aqueous extract is rich in sinapic acid (13.14 mg/100 g extract), catechin (9.88 mg/100 g extract), neohesperidin (7.38 mg/100 g extract), and chlorogenic acid (3.64 mg/100 g extract). The aqueous extract effectively scavenged toward 2,2-diphenyl-1-picrylhydrazyl (90.63%), hydrogen peroxide (98.13%), and hydroxyl radical (59.62%) at 2.0 mg/mL, and also showed high reducing power. In cytotoxic evaluation, the aqueous extract exhibited no significant cytotoxicity in human fibroblast, and it also exhibited appreciable suppression of intracellular reactive oxygen species and inhibition of lipid peroxidation. In addition, the aqueous extract upregulated the level of glutathione in a dose-dependent manner. Taken together, the aqueous extract of Arisaema cum Bile could be considered as a potential natural source that may be useful for curing diseases arising from oxidative deterioration.