Photoelectrochemical (PEC) water splitting is recognized as a sustainable strategy for hydrogen generation due to its abundant hydrogen source, utilization of inexhaustible solar energy, high-purity product, and environment-friendly process. To actualize a practical PEC water splitting, it is paramount to develop efficient, stable, safe, and low-cost photoelectrode materials. Recently, graphitic carbon nitride (g-C₃N₄) has aroused a great interest in the new generation photoelectrode materials because of its unique features, such as suitable band structure for water splitting, a certain range of visible light absorption, nontoxicity, and good stability. Some inherent defects of g-C₃N₄, however, seriously impair further improvement on PEC performance, including low electronic conductivity, high recombination rate of photogenerated charges, and limited visible light absorption at long wavelength range. Construction of g-C₃N₄-based nanosized heteroarrays as photoelectrodes has been regarded as a promising strategy to circumvent these inherent limitations and achieve the high-performance PEC water splitting due to the accelerated exciton separation and the reduced combination of photogenerated electrons/holes. Herein, we summarize in detail the latest progress of g-C₃N₄-based nanosized heteroarrays in PEC water-splitting photoelectrodes. Firstly, the unique advantages of this type of photoelectrodes, including the highly ordered nanoarray architectures and the heterojunctions, are highlighted. Then, different g-C₃N₄-based nanosized heteroarrays are comprehensively discussed, in terms of their fabrication methods, PEC capacities, and mechanisms, etc. To conclude, the key challenges and possible solutions for future development on g-C₃N₄-based nanosized heteroarray photoelectrodes are discussed.