Edible green algal seaweeds, namely Ulva intestinalis and Ulva lactuca constitute a significant repository of popular herbal medicines in the Traditional Chinese Medicinal system. The present study aimed to assess the anticancer potential of these algal members and its mode of action in cervical cancer cells SiHa. The algal samples primarily extracted in methanol was fractionated into hexane, chloroform, and aqueous algal fractions, which inhibited the proliferation of SiHa cells in a dose-dependent manner, with the algal chloroform fractions harbouring the lowest IC50 dose of 141.38 µg/ml in U. intestinalis and 445.278 µg/ml in U. lactuca. These algal chloroform fractions when studied for their in-depth mode of action, were found to damage and pulverise the nuclei, resulting in a concomitant increase in subG0-phase of SiHa cells, studied by flow cytometry. The algal treatment also caused an increase in the number of acidic vesicles and enhanced the expression of LC3BII, p62 and atg12 proteins, which together pointed out autophagy as the induced mode of cell death. Upregulated Bax and p53 expression along with decreased Bcl2 expression also correlated to autophagic cell death. Decreased expression of E6 viral oncogene was noted as a significant response to algal fractions. Lastly, these potent algal fractions when characterised pharmacologically through GC–MS analysis were found to be rich in unsaturated fatty acids, majorly palmitic acid. Hence, this study concludes that the two species of Ulva successfully decreased the proliferation of SiHa cervical cancer cells through autophagy, hinting at palmitic acid being the major responsible bioactive compound in both. © 2021, King Abdulaziz City for Science and Technology.