Abstract Unfolded protein responses, including induction of stress sensor kinases, chaperones, and apoptotic mediators, are involved in the familial amyotrophic lateral sclerosis (ALS) model related to mutant Cu/Zn superoxide dismutase (SOD1) and sporadic ALS. We hypothesized that the endoplasmic reticulum-resident factor Derlin-1 plays a pivotal role in the regulation of misfolded proteins evoked by mutant SOD1. We show that Derlin-1 overexpression reduced mutant SOD1-induced cell toxicity and increased cell viability by suppressing the activation of the ER stress pathway factors: immunoglobulin-binding protein, activating transcription factor 6 p50, and C/EBP homologous protein. Interestingly, exogenous Derlin-1 resulted in a decrease in the amount of mutant SOD1, and a lesser decrease in that of wild-type SOD1, in transfected cells. Reduced SOD1 protein expression was observed in the microsomal fraction of wild-type and mutant SOD1 cells. Our results indicate that Derlin-1 regulates the turn over of SOD1 by promoting the proteasomal and autophagosomal degradation of SOD1 protein, but not by decreasing mutant SOD1 mRNA levels. Insights into the effects of Derlin-1 on mutant SOD1 may facilitate advancements in the treatment of motor neuron degeneration associated with ALS.