To improve the photocatalytic performance of KNbO₃ for the decomposition of water into hydrogen and oxygen, the electronic structure of KNbO₃ should be modified to have a suitable bandgap with band edge positions straddling the water redox level so as to sufficiently absorb visible light. Hybrid density functional theory has been used to calculate the electronic structures of pure, N-, Mo-, and Cr-monodoped, and Mo–N and Cr–N codoped KNbO₃. In particular, the influence of the relative positions of Mo–N or Cr–N codopants on the electronic structure of KNbO₃ is discussed in detail to account for the possible difference in the photocatalytic activity of the codoped samples prepared by different experimental techniques. The defect formation energy calculations indicate that a N-doped system is difficult to form under any conditions and the codoped systems are energetically favorable under Nb-poor and O-rich conditions. It is interesting to find that the effective bandgap and stability for codoped systems decrease with the increase of the dopant concentration and/or the distance between dopants. Furthermore, the suitable bandgap and band edge position with respect to the water redox level make the Mo–N codoped systems good candidates for visible light photocatalytic decomposition of water to generate hydrogen.