Stress proteins located in the cytosol or endoplasmic reticulum (ER) participate in maintenance of cell homeostasis and development of tolerance against severe insults. In neurodegenerative diseases, a number of chaperones can ameliorate accumulation of misfolded proteins triggered by oxidative/nitrosative stress or mutated gene products. Here we show that one such ER chaperone, protein-disulfide isomerase (PDI), is S-nitrosylated in brains manifesting sporadic Parkinson's or Alzheimer's disease. Nitric oxide-induced S-nitrosylation of PDI inhibits its enzymatic activity, leads to accumulation of polyubiquitinated proteins, and activates the unfolded protein response (UPR). S-Nitrosylation also abrogates PDI-mediated attenuation of neuronal cell death triggered by ER stress or proteasome inhibition. Thus, PDI prevents neurotoxicity associated with ER stress and protein misfolding, but nitric oxide blocks this protective effect in neurodegenerative disorders via PDI S-nitrosylation.