This chapter discusses the computational design of inorganic photoelectrochemical (PEC) materials. The electronic structure of photoanodes and photocathodes significantly affect photoabsorption, carrier transport, and water redox properties. Adjusting the band edge positions with respect to redox potentials is, therefore, an important task for the design of photocatalysts for PEC application. We present our recent attempts on the band edge engineering of semiconductor photocatalysts using density functional theory (DFT) calculations. First, we discuss the effects of doping on visible light absorption as well as introduction of conductive carriers. Second, we demonstrate how the band edge positions are controlled by biaxial strain. Third, we present our studies on an evolutional crystal structure search for predicting novel photofunctional materials. These results demonstrate that modern DFT-based computational materials science is a powerful tool for finding promising PEC materials.