We demonstrate the rational synthesis of monodisperse ultrasmall platinum (Pt) nanocrsytals (NCs), in aqueous solution at room temperature, with specifically selected peptide molecules. The specific Pt-binding peptide, selected using a phage display technique, can function as a stabilizer to regulate Pt crystal nucleation and growth and, therefore, control both the morphology and size of the final Pt NCs. Uniform near-spherical Pt NCs with sizes ranging from 1.73 to 3.54 nm were achieved with a very narrow size distribution. The peptide-stabilized Pt NCs can be dispersed well in water for months. It was also demonstrated that the strong binding of peptides to the Pt NC surface is reversible by either pH modulation or peptide photolysis.