Parkinson's disease is characterized by the loss of dopaminergic neurons in the nigrostriatal pathway accompanied by the presence of intracellular cytoplasmic inclusions, termed Lewy bodies. Fibrillized α‐synuclein forms the major component of Lewy bodies. We reported a specific interaction between rat α‐synuclein and tat binding protein 1, a subunit of PA700, the regulatory complex of the 26S proteasome. It has been demonstrated that PA700 prevents the aggregation of misfolded, nonubiquinated substrates. In this study, we examine the effect of PA700 on the aggregation of wild‐type and A53T mutant α‐synuclein. PA700 inhibits both wild‐type and A53T α‐synuclein fibril formation as measured by Thioflavin T fluorescence. Using size exclusion chromatography, we present evidence for a stable PA700–α‐synuclein complex. Sedimentation analyses reveal that PA700 sequesters α‐synuclein in an assembly incompetent form. Analysis of the morphology of wild‐type and A53T α‐synuclein aggregates during the course of fibrillization by electron microscopy demonstrate the formation of amyloid‐like fibrils. Secondary structure analyses of wild‐type and A53T α‐synuclein assembled in the presence of PA700 revealed a decrease in the overall amount of assembled α‐synuclein with no significant change in protein conformation. Thus, PA700 acts on α‐synuclein assembly and not on the structure of fibrils. We hypothesize that PA700 sequesters α‐synuclein oligomeric species that are the precursors of the fibrillar form of the protein, thus preventing its assembly into fibrils.