Magnetite nanoparticles for biomedical applications are typically prepared using the coprecipitation technique, which is the most convenient method. However, the reaction pathways leading to the production of the magnetite phase in the coprecipitation reaction are not fully understood, despite the fact that the reaction path may be of significant importance in controlling the crystal structure, morphology, and particle size of the magnetite nanoparticles. In the present study, we identified the reaction pathways in the coprecipitation of magnetite; when base was slowly added to an iron chloride solution, akaganeite nucleated and transformed through goethite to magnetite. At high addition rates, an additional pathway in which ferrous hydroxide nucleated and transformed through lepidocrocite to magnetite competed with the former pathway. This difference was due to the pH inhomogeneity in the reaction medium that was present before homogeneous mixing. In most coprecipitation reactions, these magnetite formation pathways coexist, but the dominant process is the topotactic transformation of goethite to magnetite, mediated by arrow-shaped nanoparticles. The morphology of the arrow-shaped nanoparticles was explained on the basis of specific crystallographic relationships among the iron oxide phases. The proposed reaction scheme for magnetite coprecipitation could assist in devising a more detailed study of the reaction mechanism.