This work presents a facile one-step protocol for the gram-scale synthesis of iron oxide nanocubes with adjustable sizes ranging from 13 to 20 nm and with size distributions between 7 and 12%. As X-ray diffraction indicated the initial formation of the wustite phase, a formation mechanism of the nanocubes based on the wustite crystal structure is proposed. When exposed to ambient conditions, the nanoparticles rapidly oxidize to magnetite/maghemite with a remaining wustite core. The cubic morphology is attributed to the thermodynamic stability of the exposed {100} facets and the control over the growth rate via the use of a sodium oleate/oleic acid mixed ligand system. In contrast to previously reported procedures, the described synthetic approach does not require the initial preparation and isolation of iron oleate. Therefore, the amount of work and the consumption of hazardous solvents are significantly reduced. Thus, the method presented is much more efficient and environmentally more friendly while maintaining excellent control over the particles' shape, size, and size distribution.