The endoplasmic reticulum (ER) is critical in aiding cells in ensuring that proteins are folded and processed correctly, particularly during stressful situations. ER oxidoreductin-1 alpha (ERO1α) is an enzyme that is responsible for the formation of disulfide bonds during protein folding, along with protein disulfide isomerase (PDI). This redox pathway is often highly upregulated in cancer cells, allowing tumors to survive harsh conditions such as hypoxia and nutrient deprivation. This review discusses the role of the ERO1α–PDI system in cancer development through the regulation of oxidative stress, redox homeostasis, and tumor plasticity. It further shows the therapeutic potential of interrupting the ERO1α–PDI axis, which could lead to protein misfolding; enhanced generation of reactive oxygen species (ROS); and, eventually, cancer cell death.