Despite having valuable and novel metabolites, the marine microalgae species are still not thoroughly investigated for their pharmaceutical and nutraceutical importance. Therefore, this study was focused on investigating the crude extracts of marine green microalgae species, Tetraselmis sp., Nannochloropsis sp., and diatoms Chaetoceros sp., and Thalassiosira sp., isolated from the Malaysian coastal region in terms of their antioxidant activity, total phenolics, total flavonoid contents and cytotoxicity against human breast cancer cells, MCF-7. Among twenty-eight crude extracts, Tetraselmis ethanol and ethyl acetate extract showed the highest amount of total phenolic (19.87 mg GAE/g), and total flavonoid content (38.58 mg QE/g of extract), respectively. From the antioxidant assays, methanol and ethyl acetate extract of Tetraselmis sp. exhibited significantly higher (p