AbstractAggregation of α‐synuclein (α‐syn) into amyloid is the pathological hallmark of several neurodegenerative disorders, including Parkinson disease, dementia with Lewy bodies, and multiple system atrophy. It is widely accepted that α‐syn aggregation is associated with neurodegeneration, although the mechanisms are not yet fully understood. Therefore, the inhibition of α‐syn aggregation is a potential therapeutic approach against these diseases. This study used the photocatalyst for α‐syn photo‐oxygenation, which selectively adds oxygen atoms to fibrils. Our findings demonstrate that photo‐oxygenation using this photocatalyst successfully inhibits α‐syn aggregation, particularly by reducing its seeding ability. Notably, we also discovered that photo‐oxygenation of the histidine at the 50th residue in α‐syn aggregates is responsible for the inhibitory effect. These findings indicate that photo‐oxygenation of the histidine residue in α‐syn is a potential therapeutic strategy for synucleinopathies.