A green approach is proposed for the synthesis of Pt nanoparticles (PtNPs) in the bovine serum albumin (BSA) scaffold through biomineralization. The resulting BSA–PtNPs have been proved to function as peroxidase mimics that can catalyze the reaction of various peroxidase substrates in the presence of H2O2. Kinetic studies indicate that BSA–PtNPs have a much higher affinity for H2O2 than the NPs of other Pt-based peroxidase mimics. Furthermore, the BSA shell plays an important role in promoting the robust stability of PtNPs. Even in high ionic strength environment (2 M NaCl), the catalytic activity can be well preserved. These excellent properties make the BSA–PtNPs an ideal candidate for a wide range of potential applications as peroxidase mimics.