Iron oxide nanoparticles are a kind of important biomedical nanomaterials. Although their industrial‐scale production can be realized by the conventional coprecipitation method, the controllability of their size and morphology remains a huge challenge. In this study, a kind of synthetic polypeptide Mms6‐28 which mimics the magnetosome protein Mms6 is used for the bioinspired synthesis of Fe3O4 nanoparticles (NPs). Magnetosomes‐like Fe3O4 NPs with uniform size, cubooctahedral shape, and smooth crystal surfaces are synthesized via a partial oxidation process. The Mms6‐28 polypeptides play an important role by binding with iron ions and forming nucleation templates and are also preferably attached to the [100] and [111] crystal planes to induce the formation of uniform cubooctahedral Fe3O4 NPs. The continuous release and oxidation of Fe2+ from pre‐formed Fe2+‐rich precursors within the Mms6‐28‐based template make the reaction much controllable. The study affords new insights into the bioinspired‐ and bio‐synthesis mechanism of magnetosomes.