The distinguished electronic and optical properties of lead halide perovskites (LHPs) make them good candidates for active layer in optoelectronic devices. Integrating LHPs and two-dimensional (2D) transition metal dichalcogenides (TMDs) provides opportunities for achieving increased performance in heterostructured LHPs/TMDs based optoelectronic devices. The electronic structures of LHPs/TMDs heterostructures, such as the band offsets and interfacial interaction, are of fundamental and technological interest. Here CsPbBr₃ and MoSe₂ are taken as prototypes of LHPs and 2D TMDs to investigate the band alignment and interfacial coupling between them. Our GGA-PBE and HSE06 calculations reveal an intrinsic type-II band alignment between CsPbBr₃ and MoSe₂. This type-II band alignment suggests that the performance of CsPbBr₃-based photodetectors can be improved by incorporating MoSe₂ monolayer. Furthermore, the absence of deep defect states at CsPbBr₃/MoSe₂ interfaces is also beneficial to the better performance of photodetectors based on CsPbBr₃/MoSe₂ heterostructure. This work not only offers insights into the improved performance of photodetectors based on LHPs/TMDs heterostructures but it also provides guidelines for designing high-efficiency optoelectronic devices based on LHPs/TMDs heterostructures.