Magnetotactic bacteria align along the Earth’s magnetic field using an organelle called the magnetosome, a biomineralized magnetite(Fe(II)Fe(III)2O4) or greigite (Fe(II)Fe(III)2S4) crystal embedded in a lipid vesicle. Although the need for both iron(II) and iron(III) is clear, little is known about the biological mechanisms controlling their ratio 1. Here we present the structure of the magnetosomeassociated protein MamP and find that it is built on a unique arrangement of a self-plugged PDZ domain fused to two magnetochrome domains, defining a new class of c-type cytochrome exclusively found in magnetotactic bacteria. Mutational analysis, enzyme kinetics, cocrystallization withiron(II) and anin vitroMamP-assisted magnetite production assay establish MamP as an iron oxidase that contributes to the formation ofiron(III) ferrihydrite eventually required for magnetite crystal growth in vivo. These results demonstrate the molecular　　mechanisms ofironmanagement taking placeinside themagnetosome and highlight the role of magnetochrome in iron biomineralization.