Rapid and uncontrollable cell proliferation, altered metabolism, and abnormal vasculature of cancer cells make them hypoxic and result in the generation of reactive oxygen species (ROS), causing oxidative stress. Hypoxia-mediated oxidative stress represents a significant barrier to effective cancer treatment. miRNAs are emerging as a potential regulator of hypoxia-responsive genes and hypoxia-mediated oxidative stress. Based on the role of miR-140-5p in regulating a hypoxia-responsive gene, this study is aimed at understanding the miR-140-5p role in regulating hypoxia-mediated oxidative stress under breast tumor hypoxia. We found that the miR-140-5p might control the hypoxia-mediated ROS generation by regulating the Nrf2 expression. Knowing the significance of miR-140-5p in regulating hypoxia-mediated oxidative stress and breast tumor progression, targeting miR-140-5p might represent a promising strategy for anti-breast cancer therapy.