Accumulation of unfolded proteins within the endoplasmic reticulum (ER) lumen induces ER stress. Eukaryotic cells possess the ER quality control systems, the unfolded protein response (UPR), to adapt to ER stress. IRE1 alpha is one of the ER stress receptors and mediates the UPR. Here, we identified ubiquitin specific protease (USP) 14 as a binding partner of IRE1 alpha. USP14 interacted with the cytoplasmic region of IRE1 alpha, and the endogenous interaction between USP14 and IRE1 alpha was inhibited by ER stress. Overexpression of USP14 inhibited the ER-associated degradation (ERAD) pathway, and USP14 depletion by small interfering RNA effectively activated ERAD. These findings suggest that USP14 is a novel player in the UPR by serving as a physiological inhibitor of ERAD under the non-stressed condition. (C) 2009 Elsevier Inc. All rights reserved.