Background: Cissampelos pareira (Menispermaceae) is used in folk Indian system of alternative medicine, for its analgesic, antipyretic, diuretic, antilithic, and emmenagogue properties. Objective: To evaluate Cissampelos pareira (C. pareira) for in vitro cytotoxicity and in vivo antitumor activity against Dalton's Lymphoma Ascites (DLA) cells in Swiss mice. Materials and Methods: Cissampelos pareira was successively extracted using different solvents. In vitro cytotoxicity was assessed by the MTT assay. An in vivo study was carried out in methanol extract. Twenty-four hours after intraperitoneal inoculation of the DLA cells in mice, the methanol extract of C. pariera (MECP) was administered at 200 and 400 mg/kg body weight for 14 consecutive days. On day 14, six mice were sacrificed and the rest were kept alive for assessment of increase in life-span. The antitumor effect was assessed by evaluating the packed cell volume, viable tumor cell count, increase in body weight, and increase in life-span. The hematological and serum biochemical parameters and anti-oxidant properties were assessed by estimating the superoxide dismutase (SOD), catalase (CAT), and lipid peroxidation. Results: Methanol Extract of Cissampelos pariera (MECP) showed a potent cytotoxic activity, with an IC 50 value of 95.5 μg/ml and a significant (P < 0.001) decrease in packed cell volume, viable cell count, and an increased lifespan (54 and 72%). The hematological and serum biochemical profiles were restored to normal levels in MECP-treated mice. The MECP-treated group significantly (P < 0.001) decreased SOD, lipid peroxidation, and CAT to normal. Conclusion: This study demonstrated that C. pariera exhibited significant in vitro and in vivo anti-tumor activities and that it was reasonably imputable to its increasing endogenous mechanism of antioxidant property.