Magnetite nanoparticles play a key role in the nano-industry, with crucial importance in the developing nanomedicine sector. Such particles must be homogeneous, with a consistent shape and size, due to the growing need to tailor particles to more defined faceted morphologies. Here an ethylenediamine series (H2N-(-CH2CH2N-)nH2, n = 2 (DETA), 3 (TETA), 4 (TEPA), and 5 (PEHA)), of additives have been successfully used to control the morphology of nanomagnetite produced via a green ambient co-precipitation method. Whilst DETA showed less control, TETA, TEPA and PEHA mediated the near universal synthesis of faceted particles (91-97%) suggesting a near pure octahedral population (compared to only 6% of control particles). The particle size was ≈ 22 nm for all the samples and was not affected by the addition of additives. Computational molecular dynamic modelling shows the binding to the octahedral  face to be preferred for all additives with binding to the  face unfavourable for TETA, TEPA and PEHA, showing a preference to bind and direct an octahedral morphology for these 3 additives. This is further explained by the increased numbers of interactions of the longer additives with the  surface through O and Fe in the magnetite surface bonding to H and N in the additive which is better able to lie flat on the  surface. An optimum concentration of a 1 : 125 additive : iron ion ratio was determined which shows that a relatively small quantity of a cheap, organic bioinspired amine-rich additive can have a massive impact on the morphological quality of the magnetite nanoparticles. This powerful, additive-directed, green synthesis approach could be universally applied to a vast range of nanomaterial syntheses to great impact.