<p>Researching bioactive phytochemicals for eco-friendly development of novel nanotherapeutics is gaining importance. The study synthesized Fe₃O₄ nanoparticles (Fe₃O₄-NPs) using <i>Allium cepa</i> extract which served as an efficient reducing and stabilizing agent, and hence, did not require any toxic chemical precursor. The characterization of the physicochemical properties was done by using FT-IR, TEM and FESEM methods. The results verified that there was formation of NPs with a spherical morphology and distribution of size 17–43&#xa0;nm. In addition, the FT-IR spectrum shows a distinct vibrational band at 658&#xa0;cm − 1, confirming the Fe₃O₄ nanocomposite structure. The MCF-7 breast cancer cell line was studied for the biological activity of these NPs. Fe₃O₄-NPs produce oxidative stress in cells, handle with care due to toxicity. The mechanistic study revealed that ROS generation and MDA generation cooperate to generate oxidative stress. The increase in oxidative stress led to cell death via apoptosis and ferroptosis. Quantitative analysis showed that the apoptosis rate increased from 1.66 ± 0.19% in untreated control cells to 72.74 ± 0.19% in NP-treated cells. Thus, this study confirms the green synthesis of Fe₃O₄-NPs using <i>Allium cepa</i> as a sustainable and non-toxic agent. The results demonstrate the powerful anticancer effect of these nanoparticles due to ROS-mediated apoptosis and ferroptosis, which could serve as a strong candidate for future nanotherapy.</p>