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Magnetotactic bacteria produce magnetite nanoparticles called magnetosomes at ambient conditions via a protein‐stabilized transient amorphous precursor to obtain precise control over particle size and morphology. In a bioinspired approach, such biomineralization processes are emulated, mimicking proteins involved in magnetosome formation using the positively charged analog poly‐L‐arginine. While the additive is expensive, it remains elusive whether the change in magnetite formation mechanism arises solely from the polymer's cationic nature. This study uses different mass‐produced and sustainably sourced polycations to induce the biomineralization‐reminiscent formation of magnetite nanoparticles. These findings present how to achieve control over nanoparticle size (from 10 to 159 nm) and morphology (compact and sub‐structured) as well as magnetic properties (superparamagnetic, stable‐single‐domain, vortex state) at ambient temperature and pressure using these additives. Furthermore, the formation of large nanoparticles with the addition of poly(diallyldimethylammonium chloride) (PDADMAC) at low alkalinity highlights how magnetotactic bacteria may produce magnetite nanoparticles under similar conditions. Confirming the polycations' ability to electrostatic stabilize amorphous ferrihydrite, it is anticipated that parametric in vitro studies on polymer properties will provide valuable insights into magnetite biomineralization and aid in rationally designing magnetic nanomaterials. [ABSTRACT FROM AUTHOR]