Novel and advanced photocatalytic materials for water splitting are of huge interest due to the increasing global energy crisis. In this work, MoS₂/g-C₃N₄ heterojunctions are prepared as high-performance photocatalysts for hydrogen evolution using solar energy. Benefiting from the elaborate ion exchange, uniformly dispersed MoS₂ quantum dots with controllable size and a high loading amount grow in situ on the surface of g-C₃N₄ nanosheets through a simple hydrothermal process, and tight heterostructures are obtained at the interface between the two semiconductors. The quantization of MoS₂ enhances its redox activity in water splitting, and the compact heterojunction facilitates the separation of photoinduced charge carriers, resulting in efficient photocatalytic H₂ production at the MoS₂/g-C₃N₄ heterojunction. This work highlights ion exchange as a facile and effective method for obtaining high efficiency MoS₂/g-C₃N₄ heterojunction photocatalysts.