The electronic and geometrical structure of the ground and excited states of the 3d metal monoxide anions ScO-, TiO-, VO-, CrO-, MnO-, FeO-, CoO-, NiO-, CuO-, and ZnO- were calculated using density functional theory and different formulations of generalized-gradient approximations for the exchange-correlation potential. It was found that the anion states with low- and high-spin multiplicities with respect to the groundstate spin multiplicities of the corresponding neutral parents are stable toward autodetachment of the extra electron. All the low-spin multiplicity anion states are more stable than the high-spin ones, except for that of CrO-, whose ground state appears to be a high-spin multiplicity state. Computed electron affinities of the neutral monoxides are in good agreement with the experimental values obtained by laser photoelectron spectroscopy. It is shown that purl density functional methods are generally superior to a hybrid Hartree-Fock density-functional-theory approach, except for reproducing bond rupture energies.