Spirulina, a filamentous cyanobacterium, botanists classify it as a micro alga belonging to cyanophyceae, it has a simple structure but a complex composition. Spirulina and its components have been shown to have positive benefits across a range of human health indications from malnutrition to antioxidant properties. One of its species Spirulina platensis or its extract showed therapeutic properties, such as the ability to prevent cancers, decrease blood cholesterol level, reduce nephrotoxicity of pharmaceuticals and toxic metals and provide protection against the harmful effect of radiation. Therefore, the aim of the present study is to determine antioxidant activity of Spirulina platensis water extracts and their cytotoxicity against cell lines {colon carcinoma cells (HCT116) and hepatocellular carcinoma cells (HEPG2)}. Results showed that at S. platensis water extract of 1.5 g/100 mL, highest antioxidants percentage (81.1%) and total phenolic compounds (40.45 mg g–1) were recorded. For cytotoxicity assay, the IC50 concentrations (the concentration of S. platensis that is required to cause 50% inhibition in cell lines viability) were 18.8 and 22.3 μg mL–1 for HCT116 and HEPG2 cell lines, respectively. In addition, the correlation between HCT116, HEPG2 and S. platensis water extracts was negative and significant.