Density-functional studies of structural and electronic properties of transition-metal sulfides formed by 3d transition metals, based on the local spin-density approximation and including non-local corrections to the exchange–correlation functional (generalized gradient approximation), have demonstrated the importance of magneto-volume effects and magneto-structural effects, but could not achieve full agreement with experiment. A further improvement is to consider electronic correlation effects due to tightly bound and localized d-states on the transition metal atoms. With the DFT + U method used in this work, these correlation effects are taken in account and yield improved predictions for volume, magnetic moment, exchange splitting and bandgap. For MnS the semiconducting gap is correctly predicted, and for MnS2 the high-spin AFM type-III state can be stabilized over the low-spin state. For FeS even weak correlation effects lead to better predictions for the semiconducting gap, volume and magnetic moment.