Cobalt-doped Zn(O,S)/Ga₂O₃ nanoheterojunction composites with different Co contents were synthesized and characterized. The as-prepared nanocomposite photocatalysts were tested for their capabilities to evolve hydrogen with ethanol as a hole scavenger under low power UV illumination (0.088 mW cm⁻² or approximately 1/40 UV intensity of sunlight). The highest hydrogen evolution rate of 5.10 mmol g⁻¹ h⁻¹ was achieved. It was found that the enhanced hydrogen evolution amount was 50% higher than that of pure Zn(O,S) and 14% higher than that of the Co-free Zn(O,S)/Ga₂O₃ nanocomposite prepared in our previous study. This study revealed that the role of Co was considerably crucial to enhance the efficiency of photocarrier separation and lower the electron transfer resistances on catalyst surfaces, leading to higher photoresponse current and photocatalytic activity. The photocatalytic evaluation of Co-doped Zn(O,S)/Ga₂O₃ nanocomposites with different Co contents was carried out and an appropriate photoreaction mechanism is proposed to elucidate the enhanced photocatalytic activity in this study.