Graphitic carbon nitride based heterojunction photocatalysts have gained increasing attention in producing the clean energy source of hydrogen. Coupling carbon nitride (g-C3N4) with other semiconductor materials or metals as co-catalysts is considered as an effective strategy to overcome the drawbacks of g-C3N4 such as the quick recombination of photogenerated charges. In this review, the recent research advancements in the construction of g-C3N4-based heterojunctions as well as their different charge separation/transfer mechanisms will be systematically discussed, making special emphasis on the design and fabrication of type-II, Z-scheme, S-scheme and Schottky heterojunctions and their application towards H2 generation from water splitting. Finally, a summary and some crucial issues, which should be further resolved for developing advanced g-C3N4-based heterojunction photocatalysts, are presented.