Abstract Graphdiyne (GDY), which features a highly π‐conjugated structure, direct bandgap, and high charge carrier mobility, presents the major requirements for photocatalysis. Up to now, all photocatalytic studies are performed without paying too much attention on the GDY bandgap (1.1 eV at the G 0 W 0 many‐body theory level). Such a narrow bandgap is not suitable for the band alignment between GDY and other semiconductors, making it difficult to achieve efficient photogenerated charge carrier separation. Herein, for the first time, it is demonstrated that tuning the electronic bandgap of GDY via H‐substitution (H‐GDY) promotes interfacial charge separation and improves photocatalytic H 2 evolution. The H‐GDY exhibits an increased bandgap energy ( ≈ 2.5 eV) and exploitable conduction band minimum and valence band maximum edges. As a representative semiconductor, TiO 2 is hybridized with both H‐GDY and GDY to fabricate a heterojunction. Compared to the GDY/TiO 2 , the H‐GDY/TiO 2 heterojunction leads to a remarkable enhancement of the photocatalytic H 2 generation by 1.35 times under UV–visible illumination (6200 µ mol h −1 g −1 ) and four times under visible light (670 µ mol h −1 g −1 ). Such enhancement is attributed to the suitable band alignment between H‐GDY and TiO 2 , which efficiently promotes the photogenerated electron and hole separation, as supported by density functional theory calculations.