Endocrine therapy-resistant estrogen receptor-positive (ER+) breast cancer cells often exhibit an augmented capacity to maintain endoplasmic reticulum (EnR) homeostasis under adverse conditions. Oncoprotein hepatitis B X-interacting protein (HBXIP) is a known transcriptional coactivator that promotes cancer development. However, it is unclear whether HBXIP participates in maintaining EnR homeostasis and promoting drug-resistance in ER+ breast cancer. Here, we report that tamoxifen-resistant (TmaR) breast cancer cells exhibit increased expression of HBXIP, which acts as an inactivator of the unfolded protein response (UPR) to diminish tamoxifen (TAM)-induced EnR stress. We show that HBXIP deficiency promotes EnR-associated degradation (ERAD), enhances UPR-element (UPRE) reporter activity and cellular oxidative stress, and ultimately attenuates the growth of TmaR cells in vitro and in vivo. Mechanistically, we demonstrate that HBXIP acts as a chaperone of UPR transducer inositol-requiring enzyme 1a (IRE1α) and diminishes production of reactive oxygen species (ROS) in TamR breast cancer cells. Upon loss of HBXIP expression, TAM treatment hyperactivates IRE1α and its downstream proapoptotic pathways and simultaneously induces accumulation of intracellular ROS. This elevated ROS programmatically activates the other two branches of the UPR, mediated by PKR-like ER kinase (PERK) and activating transcription factor 6α (ATF6α). Clinical investigations and Kaplan-Meier plotter analysis revealed that HBXIP is highly expressed in TamR breast cancer tissues. Furthermore, reinforced HBXIP expression associated with a high recurrence and poor relapse-free survival rates in tamoxifen monotherapy ER+ breast cancer patients. These findings indicate that HBXIP is a novel regulator of EnR homeostasis and a potential target for TamR breast cancer therapy.Copyright © 2022 The Authors. Published by Elsevier Inc. All rights reserved.