Aggregation of -synuclein (SN) is implicated in neuronal degeneration in Parkinson’s disease and has prompted searches for natural compounds inhibiting SN aggregation and reducing its tendency to form toxic oligomers. Oil from the olive tree (Olea europaea L.) represents the main source of fat in the Mediterranean diet and contains variable levels of phenolic compounds, many structurally related to the compound oleuropein. Here, using SN aggregation, fibrillation, size-exclusion chromatography–multiangle light scattering (SEC-MALS)based assays, and toxicity assays, we systematically screened the fruit extracts of 15 different olive varieties to identify compounds that can inhibit SN aggregation and oligomer toxicity and also have antioxidant activity. Polyphenol composition differed markedly among varieties. The variety with the most effective antioxidant and aggregation activities, Koroneiki, combined strong inhibition of SN fibril nucleation and elongation with strong disaggregation activity on preformed fibrils and prevented the formation of toxic SN oligomers. Fractionation of the Koroneiki extract identified oleuropein aglycone, hydroxyl oleuropein aglycone, and oleuropein as key compounds responsible for the differences in inhibition across the extracts. These phenolic compounds inhibited SN amyloidogenesis by directing SN monomers into small SN oligomers with lower toxicity, thereby suppressing the subsequent fibril growth phase. Our results highlight the molecular consequences of differences