Developing cheap, highly efficient and stable electrocatalysts for both oxygen and hydrogen evolution reactions (OER and HER) is extremely meaningful to realize large-scale implementation of water splitting technology. Herein, we report the phase and composition controlled synthesis of cobalt sulfide (CoS_x) hollow nanospheres (HNSs) and their catalytic efficiencies for hydrogen and oxygen evolution reactions in alkaline media. Three CoS_x compounds, i.e., Co₉S₈, Co₃S₄, and CoS₂ HNSs, were precisely synthesized by simply adjusting the molar ratio of carbon disulfide to cobalt acetate using a facile solution-based strategy. Electrochemical results reveal that the as-prepared CoS₂ HNSs exhibit superior OER and HER catalytic performance to Co₉S₈ and Co₃S₄ HNSs in 1.0 M KOH, with overpotentials of 290 mV for the OER and 193 mV for the HER at 10 mA cm⁻², and the corresponding Tafel slopes of 57 and 100 mV dec⁻¹, respectively. In addition, the CoS₂ HNSs exhibit remarkable long-term catalytic durability, which is even superior to precious metal catalysts of RuO₂ and Pt/C. Moreover, an alkaline electrolyzer assembled using CoS₂ HNSs as both anode and cathode materials can achieve 10 mA cm⁻² at a low cell voltage of 1.54 V at 60 °C with a faradaic efficiency of 100% for overall water splitting. Further analysis demonstrates that the surface morphology, crystallographic structure and coordination environment of Coⁿ⁺ active sites in combination determine the HER/OER activities in the synthesized binary CoS_x series, which would provide insight into the rational design of transition metal chalcogenides for highly efficient hydrogen and oxygen-involved electrocatalysis.