Nanohybrid catalysts hold great promise for photocatalysis and photoelectrocatalysis, with significant progress still to be made. We synthesize a graphitic carbon nitride (GCN)–CeO2 heterojunction via electrostatic self-assembly. Characterization confirms that CeO2 nanocubes are uniformly anchored onto layered GCN, forming a high-quality interface with abundant active sites. This architecture facilitates efficient separation of photogenerated charge carriers and an improved optical response, as further supported by density functional theory and finite-difference time-domain simulations, which reveal a modified band structure and optical response at the type-II heterojunction interface. The resulting hybrid exhibits excellent water splitting performance, with a photocurrent density of 5.70 mA cm–2 at a low onset potential of 0.43 V vs Ag/AgCl. The GCN–CeO2 photocatalyst shows an enhanced hydrogen evolution rate of 809.23 μmol g–1 h–1, which is 6.7 times higher than that of pure CeO2 and 3.2 times higher than that of the GCN photocatalyst. The reported findings highlight the promising potential of electrostatic self-assembly as an effective strategy for the development of efficient catalysts for solar fuel production.