The geometric structure, electronic states, and surface spin polarization of a (H, NO)-coadsorbed Fe3O4(100) surface have been studied using density functional theory calculations. H atoms saturate the surface dangling bonds through bonding with the O atom (O1) without a tetrahedral Fe neighbor (Fe(A)), inducing a deeper level shift of the spin-up surface state bands and a widening of the spin-up band gap between the Fermi level (EF) and the valence band maximum. NO molecules are adsorbed on surface octahedral Fe atoms (Fe(B)). The adsorbate/substrate and molecule–molecule interactions cause considerable filling and broadening of the spin-down 2π* states of the adsorbed NO molecule. A −100% spin polarization is obtained over the energy range of −0.8 eV to EF for the (H, NO)-coadsorbed Fe3O4(100) surface, indicating that this system has greater potential for application in spintronic devices than either the solely H-adsorbed or NO-adsorbed surfaces. Furthermore, the adsorbed NO molecule can provide a considerable density of −100% spin-polarized states. Both of these findings are significant for the application and design of spintronic devices.