Protein misfolding diseases are a group of devastating disorders characterized by structural conversion of a soluble protein into an amyloid-like aggregate. Typically, the structural conversion occurs by misfolding of a single disease-associated protein, such as α-synuclein (αS) in Parkinson’s disease, amyloid-β in Alzheimer’s disease, and prion protein (PrP) in transmissible spongiform encephalopathies (TSEs). However, accumulating evidence has implicated that cross-interactions between heterologous amyloidogenic proteins dramatically impact on amyloidogenesis and disease pathology. Here we show αS in a monomeric state can suppress amyloidogenesis of PrP in vitro. Thioflavin-T assays and transmission electron miscopy revealed that monomeric αS inhibits the nucleation step of amyloidogenesis without inhibiting the growing step. Surface plasmon resonance and co-sedimentation assays neither detected interaction between αS and monomeric PrP nor fibrillar PrP. These results suggested that αS suppress amyloidogenesis of PrP by binding to a transiently accumulated intermediate, such as a partially unfolded state. Moreover, we found that oligomeric αS, which was recently suggested to interact with PrP, also did not interact with PrP. Taken together, our study revealed a chaperon-like activity of αS against PrP amyloidogenesis, suggesting a possible involvement of αS in the pathology of TSEs.