Accumulation of aberrant proteins in the endoplasmic reticulum (ER) triggers the unfolded protein response pathway that helps the cell to survive under these stress conditions. Herp is a mammalian ubiquitin domain protein, which is strongly induced by the unfolded protein response. It is involved in ER-associated protein degradation (ERAD) and interacts directly with the ubiquitin ligase Hrd1, which is found in high molecular mass complexes of the ER membrane. Here we present the first evidence that Herp regulates Hrd1-mediated ubiquitylation in a ubiquitin-like (UBL) domain-dependent manner. We found that upon exposure of cells to ER stress, elevation of Herp steady state levels is accompanied by an enhanced association of Herp with pre-existing Hrd1. Hrd1-associated Herp is rapidly degraded and substituted by de novo synthesized Herp, suggesting a continuous turnover of the protein at Hrd1 complexes. Further analysis revealed the presence of multiple Hrd1 copies in a single complex enabling binding of a variable number of Herp molecules. Efficient ubiquitylation of the Hrd1-specific ERAD substrate a1-antitrypsin null Hong Kong (NHK) required the presence of the Herp UBL domain, which was also necessary for NHK degradation. In summary, we propose that binding of Herp to Hrd1-containing ERAD complexes positively regulates the ubiquitylation activity of these complexes, thus permitting survival of the cell during ER stress.