AbstractBand‐gap engineering of oxide materials is of great interest for optoelectronics, photovoltaics, and photocatalysis applications. In this study, electronic structures of perovskite oxynitrides, LaTiO2N and SrNbO2N, and solid solutions, (SrTiO3)1−x(LaTiO2N)x and (SrTiO3)1−x(SrNbO2N)x, are investigated using hybrid density functional calculations. Band gaps of LaTiO2N and SrNbO2N are much smaller than that of SrTiO3 owing to the formation of a N 2p band, which is higher in energy than the O 2p band. The valence‐ and conduction‐band offsets of SrTiO3/LaTiO2N and SrTiO3/SrNbO2N are computed, and the adequacy for H2 evolution is analyzed by comparing the positions of the band edges with respect to the standard hydrogen electrode (SHE). The band gap of (SrTiO3)1−x(LaTiO2N)x and (SrTiO3)1−x(SrNbO2N)x solid solutions are also discussed.