To harvest solar energy for water splitting and produce pollution-free hydrogen and oxygen, high-performance photocatalysts are essential. Here, by combining different two-dimensional (2D) group III–V MX (M = Ga, In and X = P, As) monolayers, we designed 144 van der Waals (vdW) heterostructures to identify efficient photoelectrochemical materials. Using first-principles calculations, we investigated the stabilities, electronic properties, and optical properties of these heterostructures. After a careful screening process, we elected GaP/InP in a BB-II stacking configuration as the most promising candidate. This specific GaP/InP configuration has a type-II band alignment with a gap value of 1.83 eV. The conduction band minimum (CBM) is located at −4.276 eV, and the valence band maximum (VBM) is located at −6.217 eV, fully satisfying the requirements of the catalytic reaction under pH = 0. Additionally, light absorption has been improved through the construction of the vdW heterostructure. These results could help in understanding the properties of the III–V heterostructures and guide the experimental synthesis of these materials for photocatalysis applications.