Self-assembled peptide nanostructures with actively folded secondary structures have potential to mimic the function of proteins. We here show that alpha-helix-stabilized self-assembled peptide nanostructures (alpha SSPNs), whose sizes are comparable to those of proteins, have potential to be developed as protein-protein interaction (PPI) inhibitors along with several unprecedented properties. Using p53-MDM2 PPI as a model system, the molecular recognition and modulation of PPIs by alpha SSPN grafted with a p53 alpha-helix (p53 alpha SSPN) were investigated. The competition assay showed that the p53 alpha SSPN can inhibit the p53-MDM2 interaction. Interestingly, the p53 alpha SSPN was far more resistant to degradation by the protease chymotrypsin than the monomeric p53 peptide and had high thermal stability. These results suggest that the alpha SSPN scaffold holds great potential to be developed as a novel class of PPI inhibitors.