Two-dimensional van der Waals heterostructures have been widely designed and applied to numerous optoelectronic devices such as photoelectrochemical (PEC)-type photodetectors, water splitting, and solar cells. The understanding of the influence of band alignment in type-I heterostructures on the photoelectric response remains incomplete yet essential for designing new optoelectronic devices. Herein, two-step physical vapor deposition is used to construct a type-I SnSe₂/ZnS heterostructure, which is confirmed by ultraviolet photoelectron spectroscopy and X-ray photoelectron spectroscopy. The type-I heterostructure-based PEC-type photodetector exhibits excellent photoresponse, high stability, and high sensitivity in the ultraviolet-visible range. Furthermore, the photoresponsivity of SnSe₂/ZnS is up to 172.60 µA W⁻¹, which is 7.4- and 2.0-fold larger than that of pure ZnS and SnSe₂, respectively. Moreover, the SnSe₂/ZnS heterostructure possesses high photoelectrocatalytic activity in water splitting, and the total hydrogen production within 2 h is up to 81.25 µmol cm⁻². The high PEC-type photodetector and water splitting performances in SnSe₂/ZnS are due to the synergistic effect of high light utilization and efficient charge transport. Our work provides a new method for improving photoelectric response by forming type-I heterostructures and for designing high-performance optoelectronic devices for photodetectors and water splitting.