Shape-controlled synthesis requires rigorous kinetic control over both nucleation and growth. For platinum (Pt), to date it is still challenging to generate twinned seeds in nucleation in a controllable fashion. Here, we report that a specific Pt binding peptide BP7A is able to mediate and stabilize single-twinned seeds formation at the nucleation stage under mild conditions. Importantly, it targets the control over nucleation directly. Combining with control over growth, we further demonstrate the rational design and synthesis of single-twinned structures, right bipyramid and {111}-bipyramid, by incorporating targeted facet stabilization over {100} facet and {111} facet, respectively. To the best of our knowledge, this is the first report on the successful synthesis of single-twinned bipyramids for Pt nanocrystals (NCs) with high yields. The work here demonstrates the power of biomolecules in recognizing and mediating inorganic nanomaterials synthesis, guiding the formation of material structures that are otherwise unconventional, and hence presenting one step further toward predictable and programmable biomimetic synthesis.