Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder characterized by progressive degeneration of motor neurons. TDP-43 has been found to be a major component of ubiquitin-positive inclusions in ALS. Aberrant TDP-43, which is found in inclusions, is phosphorylated and is re-distributed from the nucleus to the cytoplasm. Alterations of TDP-43 protein, particularly insolubilization/aggregation and cytosolic distribution are thought to be involved in the pathogenesis of ALS. Levels of 4-hydroxynonenal (HNE), a marker of oxidative stress, have been reported to be elevated in sporadic ALS patients. However, the effects of HNE on TDP-43 are unclear. In this study, we found that HNE treatment of cells causes insolubilization, phosphorylation, and partial cytosolic localization of TDP-43. HNE-induced cytosolic TDP-43 was diffusely localized and only a small proportion of TDP-43 localized to stress granules, which are transient structures. HNE-induced TDP-43 insolubilization and phosphorylation were even observed 24 h after washout of HNE. We also showed that the cysteine residues of TDP-43 are responsible for HNE-induced insolubilization of TDP-43. Our results indicate that HNE can cause biochemical changes of TDP-43, which resemble the aberrant alterations of this protein in ALS, and suggest that upregulation of HNE could be a risk factor for ALS.