We present a synthetic protocol for the solution-phase synthesis of monocrystalline, metallic iron nanoparticles based on seed-mediated growth, showing near-single nanometre control over particle size. A shape evolution to cubic nanoparticles is also observed with increasing size. Magnetic properties were measured after surface oxidation, showing the potential of our protocol to tune the magnetism of iron nanoparticles for applications requiring superparamagnetic or ferromagnetic nanoparticles.