Abstract Perfect truncated octahedral magnetite crystals were successfully synthesized from FeCl3·6H2O and poly(acrylic acid) (PAA) in an alkaline ethylene glycol solution via a mild one-step solvothermal process. The structure and morphologies of the obtained products were characterized by powder X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The effects of synthetic parameters including reaction temperature, reaction time, and the amount of alkali on the morphological evolution of the truncated octahedral magnetite crystals were systematically explored. 160°C was found to be the optimum temperature for the formation of truncated octahedral magnetite crystals. Below 160°C, little magnetite was formed. Above 160°C, the truncated octahedrons were gradually transformed into irregular-shaped polyhedrons. Alkali is indispensable in promoting the formation of magnetite at mild temperatures. The truncated octahedral magnetite crystals were found to be ferromagnetic and had a saturation magnetization of about 83emu/g.