Herein, transition metal chalcogenides of pristine cobalt sulfides are rationally designed to act as robust bifunctional photocatalysts for visible-light-driven water splitting for the first time. Through moderate solvothermal route, cobalt sulfides are synthesized in situ growth and observed by scanning electron microscope image analysis. Noteworthily, 3D hierarchical cobalt sulfides acting as bifunctional photocatalysts are implemented to catalyze the visible-light-driven oxygen evolution reaction and hydrogen evolution reaction. This efficient, earth-abundant, and nonnoble water splitting catalyst for artificial photosynthesis is thoroughly analyzed by various spectroscopic techniques with the aim of investigating its photocatalytic mechanism under visible-light illumination. The main catalyst of CoS-2 exhibits considerable H₂ evolution rate of 1196 µmol h⁻¹ g⁻¹ and O₂ yield of 63.5%. The efficient activity is attributed to the effective electron transfer between the photosensitizer and catalyst, which is verified by transient absorption experiments. The effective electron transfer between the photosensitizer and catalyst during water oxidation is verified by the dramatic decline of [Ru(bpy)₃]³⁺ concentration in the presence of the catalyst CoS-2. At the same time, transient absorption experiments support a rapid electron transfers from ³EY* (excited photosensitizer eosin-Y) to the catalyst CoS-2 for efficient hydrogen evolution.