Magnetite nanocrystals were synthesized in the presence of a protein involved in the biomineralization of bacterial magnetosomes, recombinant Mms6; the mammalian iron-storage protein, ferritin; and two proteins not known to bind iron, uterocalin Lcn2, and bovine serum albumin. In order to mimic the conditions at which magnetite nanocrystals are formed in magnetotactic bacteria, magnetite synthesis was performed in a polymeric gel to slow down the diffusion rates of the reagents. Recombinant Mms6 protein facilitated formation of ~30nm single-domain, uniform magnetite nanocrystals in solution, as verified by transmission electron microscopy analysis and magnetization measurements. The nanocrystals formed in the presence of ferritin, Lcn2, and bovine serum albumin did not exhibit the uniform sizes and shapes observed for those produced in the presence of Mms6. Mms6-derived magnetite nanoparticles showed the largest magnetization values above the blocking temperature, as well as the largest magnetic susceptibility compared to those of the nanomaterials synthesized with other proteins. The latter is indicative of a substantial effective magnetic moment per particle, which is consistent with the presence of magnetite with a well-defined crystalline structure. The combination of electron microscopy analysis and magnetic measurements confirm our hypothesis that Mms6 promotes the shape-selective formation of uniform superparamagnetic nanocrystals. This provides a unique bioinspired route for synthesis of uniform magnetite nanocrystals.