Formation of misfolded protein aggregates is a remarkable hallmark of various neurodegenerative diseases including Alzheimer disease, Parkinson disease, Huntington disease, prion encephalopathies, and amyotrophic lateral sclerosis (ALS). Superoxide dismutase 1 (SOD1) immunoreactive inclusions have been found in the spinal cord of ALS animal models and patients, implicating the close involvement of SOD1 aggregates in ALS pathogenesis. Here we examined the molecular mechanism of aggregate formation of ALS-related SOD1 mutants in vitro. We found that long-chain unsaturated fatty acids (FAs) promoted aggregate formation of SOD1 mutants in both dose- and time-dependent manners. Metal-deficient SOD1s, wild-type, and mutants were highly oligomerized compared with holo-SOD1s by incubation in the presence of unsaturated FAs. Oligomerization of SOD1 is closely associated with its structural instability. Heat-treated holo-SOD1 mutants were readily oligomerized by the addition of unsaturated FAs, whereas wild-type SOD1 was not. The monounsaturated FA, oleic acid, directly bound to SOD1 and was characterized by a solid-phase FA binding assay using oleate-Sepharose. The FA binding characteristics were closely correlated with the oligomerization propensity of SOD1 proteins, which indicates that FA binding may change SOD1 conformation in a way that favors the formation of aggregates. High molecular mass aggregates of SOD1 induced by FAs have a granular morphology and show significant cytotoxicity. These findings suggest that SOD1 mutants gain FA binding abilities based on their structural instability and form cytotoxic granular aggregates.