ABSTRACT Cancer is one of the major concerns in health and medicine. Studies have been done by previous researches on the medicinal value of flavonoids present in Amaranthus viridis. However, limited claims were available in terms of their anti-proliferative ability against cancer cells. Flavonoid extracts were prepared by semi-purification using ethanol and ethyl acetate. Phytochemical screening using UV-Vis spectrophotometry was used to verify the presence of flavonoids. MTT cytotoxicity assay using MCF-7 breast cancer cells was employed to determine the cytotoxic effect of the plant extract. Four concentrations of the extract of A. viridis leaves were used as treatment to cultured cells. Doxorubicin and DMSO were used as the positive and negative control, respectively. An inhibition concentration (IC50) of 18.33 ug/mL of the extract was reported after the assay, indicating that the administration of the treatment caused the death of the cells as seen when the yellow dye was not reduced into a purple formazan. Statistical analysis on the IC50 of doxorubicin and the A. viridis extract showed significant relationship, which indicated that doxorubicin has a more potent cytotoxic activity, though the extract had promising effects as seen in its IC50 being less than 30 ug/mL. These results can be attributed to the presence of the flavonoid quercetin which has been found out to possess cytotoxic effects against cancer cells. With these findings, a more natural, less toxic, cost-efficient and more readily available treatment regimen can be utilized for the management of cancer.