Kappaphycus alvarezii, marine red algae was collected from the Mandapam coastal region, Tamil Nadu, India to examine cytotoxic and redox capacity of its extracts. HPLC analysis indicates the presence of phenolic compounds in the methanolic extract. The maximum phenol content measured as mg of gallic acid standard equivalent was found to be 86.45 ± 70.72 mg/ml extract. The total flavonoid content was found to be 85.52 ± 32.57 mg/ml in the extract (using quercetin as a standard). The antioxidant potential of the extracts was determined by employing DPPH scavenging assay and antioxidant assay using the phosphomolybdenum method. Methanolic extract demonstrated efficient antioxidant capacity and DPPH radical scavenging activity. GC-MS analysis was carried out in crude extracts of K. alvarezii. MTT assay showed that the extracts are toxic to HeLa cells and their IC50 value was 1.75 mg/ml. The study shows that different extracts of K. alvarezii have strong antioxidant and less cytotoxic activity against HELA cells. © 2020 Indian Academy of Sciences.