Unfolded protein response (UPR) is an adaptive reaction that allows cancer cells to survive endoplasmic reticulum (EnR) stress that is often induced in the tumor microenvironment because of inadequate vascularization. Previous studies report an association between activation of the UPR and reduced sensitivity to antiestrogens and chemotherapeutics in estrogen receptor α (ERα)-positive and triple-negative breast cancers, respectively. ERα has been shown to regulate the expression of a key mediator of the EnR stress response, the X-box-binding protein-1 (XBP-1). Although network prediction models have associated ERβ with the EnR stress response, its role as regulator of the UPR has not been experimentally tested. Here, upregulation of wild-type ERβ (ERβ1) or treatment with ERβ agonists enhanced apoptosis in breast cancer cells in the presence of pharmacological inducers of EnR stress. Targeting the BCL-2 to the EnR of the ERβ1-expressing cells prevented the apoptosis induced by EnR stress but not by non-EnR stress apoptotic stimuli indicating that ERβ1 promotes EnR stress-regulated apoptosis. Downregulation of inositol-requiring kinase 1α (IRE1α) and decreased splicing of XBP-1 were associated with the decreased survival of the EnR-stressed ERβ1-expressing cells. ERβ1 was found to repress the IRE1 pathway of the UPR by inducing degradation of IRE1α. These results suggest that the ability of ERβ1 to target the UPR may offer alternative treatment strategies for breast cancer.