The unfolded protein response (UPR) is triggered during stress of the endoplasmic reticulum (ER) and facilitates tissue homeostasis. Considering the role of p53 tumor suppressor gene in the interpretation of stress-inducing stimuli, in this study, we explored whether p53 modulates UPR. We found that p53 ablation resulted in a profound sensitivity to tunicamycin that was associated with liver dysfunction, ground glass hepatocyte (GGH) development and nuclear atypia/dysplasia. Binding immunoglobulin protein (BiP)/glucose-regulated protein 78 (GRP78) chaperone was readily detected in the cytoplasm of GGHs, confirming ER expansion. Tunicamycin administration induced BiP/GRP78 and GRP94 expression more potently in the p53-deficient mice than in controls and elevated phosphatidylcholine, the major lipid of ER, by a p53-dependent mechanism. Furthermore, alternative splicing of XBP1, the transcription factor that executes the UPR, was more efficient in cells which do not express p53. The cytoprotective effects of p53 were confirmed by cell viability studies, indicating that p53 deficiency conferred sensitivity against tunicamycin. Our findings show that p53 protects from the hepatotoxic effects of chronic ER stress. Stimulation of p53 activity when intense UPR is undesirable may possess therapeutic implications.