Functionalizing noble metal nanoparticles is often necessary to tailor their properties, in particular for biological applications such as drug delivery, therapy and imaging. However, this process is highly challenging, due to the complications in maintaining similar geometrical parameters of the nanoparticles as of without functionalization. The heptapeptide H-Lys-[Pro-Gly-Lys]2-NH2 was found to be an efficient additive for the formation and stabilization of small and monodisperse platinum nanoparticles (PtNPs). We show that the same additive peptide can be used as a platform for the functionalization of PtNPs, as demonstrated by the introducion of three different moieties: coumarin for monitoring cellular uptake of the PtNPs; biotin for detecting their stability in human blood serum; and an arginine-containing peptide for enhanced cell penetration. The peptide-coated PtNPs can be functionalized both before and after reduction of the metal ions, in aqueus environment, emphasizing the high robustness of the system.