Brown alga, Padina boergesenii was collected from the Mandapam coast used for this present study. The phytochemical screening of the methanol extract of P. boergesenii revealed the presence of the Phenolic, flavonoids, tannins, and terpenoids. The total phenolic and flavonoid content were found to be 84.96±0.40 mg/g expressed as Gallic acid equivalent, 85.42 ± 0.97 mg/g expressed as Quercetin equivalent respectively. Antioxidant activity of the methanol extract was determined by the Phosphomolybdenum method and the scavenging activity on DPPH and Nitric oxide was evaluated. The methanol extract has shown the maximum antioxidant and scavenging activity among all other extract. Cytotoxicity of methanol extract against the liver cancer cell line Hep G2 was checked by MTT assay and the IC50 value were found to be 1.67 mg/ml. The present study has proved P. boergesenii has strong antioxidant and cytotoxic activity which may be utilized for the drug development.