jats:titleAbstract</jats:title>jats:pGreigite magnetosomes produced by magnetotactic bacteria (MTB) are widely distributed in natural environments, but large uncertainties remain regarding their magnetic biosignatures. Here, we have constructed micromagnetic models with realistic biogenic greigite particles to quantify the magnetic properties and magnetotaxis efficiency of greigite‐producing MTB cells. Our calculations suggest coercivity (jats:italicB</jats:italic>jats:subc</jats:sub>) of ∼15–21 mT for intact greigite‐producing rod‐shaped MTB and many‐celled magnetotactic prokaryotes, with jats:italicB</jats:italic>jats:subc</jats:sub> decreasing to ∼11 mT for greigite magnetofossils with clumped particles. These magnetic signatures make biogenic greigite distinguishable from typical biogenic magnetite and inorganic greigite, providing reliable magnetic criteria to detect biogenic greigite in a wide range of environmental and geological settings. Our numerical calculations suggest that rod‐shaped greigite‐producing MTB have a similar magnetotaxis efficiency to magnetite MTB, likely by biomineralizing more greigite crystals to compensate for the lower saturation magnetization of greigite and less ordered chains in greigite MTB cells, demonstrating biological‐controlled optimization of their magnetic nanostructure.</jats:p>