CYANOBACTERIA represent a promising but not totally explored source of a wide array of secondary metabolites. They also represent a source for the discovery of compounds and drugs. In the present study, three cyanobacterial species, namely, Arthrospira platensis MH285264, Leptolyngbya boryana MH155317, and Leptolyngbya sp. MH285263 were tested for their anticancer capabilities against hepatocellular carcinoma (Hep-G2) and breast adenocarcinoma (MCF-7) cell lines. In order to obtain the highest biomasses of the tested organisms, they were optimized using 8 media recipes. The optimized organisms were tested for their biochemical constituents. The GC-MS of the tested cyanobacterial extracts revealed the presence of a number of bioactive metabolites which were previously known for their anticancer activity. The extracts of A. platensis and L. sp. showed a cytotoxic effect against the Hep-G2 cell line with IC50 values of 14 and 15μg.mL−1, respectively. The extracts of A. platensis and L. sp. showed more cytotoxic effects against the MCF-7 cell line with IC50 values of 12.1 and 13.2μg.mL−1, respectively. On the basis of the undiluted concentration used in this study (100μg.mL⁻¹), the maximum inhibitory percentages compared with the control against the Hep-G2 and MCF-7 cell lines were 73.2% and 91.2%, respectively, by extracts of L. sp. and A. platensis. L. boryana exhibited a low and neglectable anticancer activity against both cell lines. The obtained promising results herald the existence of natural and safe alternatives with a high efficiency extracted from A. platensis and L. sp. for inclusion in the strategies of cancer treatment.