Titanium dioxide (TiO2), a metal oxide photocatalyst, is one of the most widely studied materials for photocatalytic applications in energy production and environmental remediation owing to its excellent photocatalytic performance. Recently, graphitic carbon nitride (g-C3N4) has gained a widespread photocatalytic application owing to its band gap of 2.70 eV, good physicochemical stability, ease of processing, and low cost. However, the photocatalytic performance characteristics of TiO2 and g-C3N4 determined individually have been confirmed by researchers to be unsatisfactory for heterogeneous photocatalysis owing to the high recombination capacity of photogenerated electron–hole pairs and the low energy conversion efficiency. A theoretical understanding of the relationship between the interfaces of TiO2 and g-C3N4 is still lacking. In this study, we investigated the effects of the interface structure on the electronic properties of the anatase TiO2/g-C3N4 heterostructure using density functional theory (DFT) calculations. The interaction between the anatase TiO2 surface and the g-C3N4 monolayer with (1 0 1)/(0 0 1) facets has been revealed, where a van der Waals heterojunction is formed. The anatase TiO2 (1 0 1)/g-C3N4 (0 0 1) heterostructure has a narrow band gap (1.876 eV) shown by DFT calculation. It effectively separates electron–hole pairs, leading to a strong optical absorption ability in the visible light region with a maximum absorption wavelength of 661 nm. The strengthened separation of electron–hole pairs and the restrained carrier recombination in the g-C3N4/TiO2 interface were analyzed on the basis of the Z-scheme photocatalytic mechanism. The results suggest that the anatase TiO2 (1 0 1)/g-C3N4 (0 0 1) heterostructure is a promising material for photocatalytic applications.