The spin polarization of magnetite Fe 3 O 4 is significantly reduced at its surfaces, which is unfavorable for the development of spintronic devices based on this material. In order to enhance the surface spin polarization, the Fe 3 O 4 (100) surface is modified here through the adsorption of boron (B) atoms and investigated using density functional theory (DFT) calculations. We find for the bulk-terminated and cation-redistributed surfaces that a band gap is opened in the spin-up electronic states due to the formation of a strong bond between the B atom and a surface oxygen atom, i.e. , B adsorption induces half-metallicity at the Fe 3 O 4 (100) surface. Besides the surface Fe and O atoms, the adsorbed B atoms have a considerable density of −100% spin-polarized electronic states at the Fermi level, which might provide a means of improving the efficiency of spin injection into spintronic devices. Spin polarization of the Fe 3 O 4 (100) surface is enhanced by B adsorption through the opening of the spin-up band gap.