Accumulation of unfolded secretory proteins in the endoplasmic reticulum (ER), namely ER stress, is hazardous to eukaryotic cells and promotes the unfolded protein response (UPR). Ire1 is an ER-located transmembrane protein that senses ER stress and triggers the UPR. According to previous in vitro experiments, 4-phenylbutyrate (4-PBA) works as a chemical molecular chaperone. Since 4-PBA attenuates the UPR in mammalian tissue cultures, this chemical may have clinical potential for restoring ER-stressing conditions. In this study, we investigated 4-PBA's mode of action using the yeast Saccharomyces cerevisiae as a model organism. Although 4-PBA blocked a dithiothreitol (DTT)-induced UPR, it did not appear to restore impairment of ER protein folding that was caused by DTT. Moreover, even under non-stress conditions, 4-PBA attenuated UPR that was induced by an Ire1 mutant that exhibits a substantial activity without sensing ER accumulation of unfolded proteins. We also found that 4-PBA drastically promotes the degradation of Ire1. These observations indicate that at least in the case of yeast cells, 4-PBA suppresses the UPR not through restoration of the ER function to correctly fold proteins. Instead, the accelerated degradation of Ire1 possibly explains the reason why the UPR is attenuated by 4-PBA.