Carbon-doped KNbO₃ photocatalyst was synthesized for the first time via a simple combination of hydrothermal and post-calcination processes with glucose as the carbon source. The synthesized sample had excellent performance in photocatalytic hydrogen production under simulated sunlight. Various techniques were applied to investigate the origin of the extraordinary photocatalytic activity. X-ray diffraction and Ar⁺ sputtering X-ray photoelectron spectroscopy analyses proved that carbon atom was successfully doped into the lattice of KNbO₃. Scanning electron microscopy and N₂ adsorption analysis indicated that the particle size of KNbO₃ was decreased due to the doping of carbon, which resulted in the increase of the BET surface area. UV–vis diffuse reflectance spectroscopy experiment verified that the carbon doping extended the light absorption region to visible light. Both the changes in surface area and optical property are beneficial to the photocatalytic reaction. However, the stronger H₂ generation rate of carbon-doped KNbO₃ was mostly attributed to the enhanced separation efficiency of electron-hole pairs due to the presence of carbon dopant. The promotion effect is closely correlated with the content of carbon dopant. C-KNbO₃ sample calcined at 350 °C displayed the highest hydrogen generation rate of 211 μmol g⁻¹ h⁻¹, which is 42 times higher than that of pure KNbO₃. Additionally, the synthesized carbon-doped KNbO₃ also presented good photoactivity under visible-light illumination. This study provides a feasible way for the synthesis of other similar photocatalysts with high efficiency.