Manipulating the ferromagnetism of diluted magnetic semiconductors by tuning the valence state of doped ions is found to be achievable in Mn-doped NiO. First-principles calculations predict that the interactions between substitutional Mn(3+) ions in NiO are ferromagnetic, while the Mn(2+)-Mn(2+) interactions are antiferromagnetic. This scenario is experimentally supported by a great enhancement of saturation magnetization with increased Mn(3+)/Mn(2+) ratio in Mn-doped and (Mn, Li)-codoped NiO.