Mutations in the gene encoding fused in sarcoma (FUS) are linked to amyotrophic lateral sclerosis (ALS), but the mechanisms by which these mutants trigger neurodegeneration remain unknown. Endoplasmic reticulum (ER) stress is increasingly recognized as an important and early pathway to motor neuron death in ALS. FUS is normally located in the nucleus but in ALS, FUS redistributes to the cytoplasm and forms inclusions. In this study, we investigated whether FUS induces ER stress in a motor neuron like cell line (NSC-34). We demonstrate that ER stress is triggered in cells expressing mutant FUS, and this is closely associated with redistribution of mutant FUS to the cytoplasm. Mutant FUS also colocalized with protein disulfide-isomerase (PDI), an important ER chaperone, in NSC-34 cells and PDI was colocalized with FUS inclusions in human ALS lumbar spinal cords, in both sporadic ALS and mutant FUS-linked familial ALS tissues. These findings implicate ER stress in the pathophysiology of FUS, and provide evidence for common pathogenic pathways in ALS linked to the ER.