Graphitic carbon nitride (g-C₃N₄) is considered as an attractive and appropriate material to split water and produce hydrogen (H₂) under visible light. Nevertheless, the H₂ evolution rate of g-C₃N₄ is low due to the rapid recombination of electron–hole pairs. In this context, we develop a novel g-C₃N₄ system combined with N-hydroxyphthalimide (denoted as NHPI) through π–π interactions and hydrogen bonds at the interface for circumventing this issue. NHPI conjugated small molecules act as an efficient hole mediator to reduce the recombination of electron–hole pairs and enhance the photocatalytic activity of g-C₃N₄. Under visible light excitation, the redox couple NHPI/NHPI⁺ serves as a redox shuttle, which can facilitate the transfer of holes from g-C₃N₄ to triethanolamine (TEOA), and the electrons can be shuttled to a Pt cocatalyst. Time-resolved photoluminescence (TRPL), photoelectrochemical (PEC) and electron paramagnetic resonance (EPR) measurements exhibit that the introduction of NHPI on g-C₃N₄ boosts the visible-light photocatalytic activity through accelerating spatial separation of the electron–hole pairs. Upon introducing optimal 2 wt% NHPI, the photocatalytic H₂ production rate of the g-C₃N₄/2 wt% NHPI (CN/2 wt% NHPI) composite is significantly enhanced up to 1145.4 μmol h⁻¹ g⁻¹, exceeding that of pristine g-C₃N₄ (274.0 μmol h⁻¹ g⁻¹) by 4.2 times. Moreover, the reusability test shows that no obvious loss of activity is observed over the CN/2 wt% NHPI sample. Overall, an excellent stable and highly efficient photocatalyst of the CN/NHPI heterostructure has been obtained, which could find potential application in solar-to-fuel conversion.