Sparked by growing pollution issues, research aiming at a better harvesting of solar energy in photocatalysts for environmental remediation has been thriving. In this study, a novel mixed valence state of Co_2.67S₄ nanoparticles with full-spectrum-responsive photocatalytic activity had been fabricated via a facile solvothermal route. The as-synthesized samples were systematically characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and UV–vis-NIR diffuse reflection spectroscopy (UV–vis-NIR DRS). The photocatalytic performance of as-obtained samples had been investigated by the degradation of methylene blue (MB) in aqueous solution. The Co_2.67S₄ nanoparticles with the particle size of 5–20 nm could degrade MB with the efficiency of 64%, 84% and 68% under the UV light, visible light and near-infrared light exposure, respectively. Furthermore, a possible photocatalytic mechanism toward the near-infrared region had been proposed to be that the Co²⁺/Co³⁺ redox couple played vital parts in the photocatalytic activity of Co_2.67S₄. This study provides a novel full solar spectrum-responsive photocatalyst for solar-light utility and environmental remediation.