The development of efficient non-noble metal hydrogen-evolving electrocatalysts is of paramount importance for sustainable hydrogen production from water. Herein, we report the direct growth of metallic Co9S8 nanosheets on carbon cloth (CC) through a facile one-pot solvothermal method. We also show that the introduction of a tiny amount of Zn2+ ions (Zn : Co mol ratio of 0.5–1 : 100) in the synthesis system can reduce the thickness, improve the crystallinity, and optimize the surface structure of Co9S8 nanosheets, without Zn-doping. Furthermore, we show that the resulting Co9S8/CC materials can serve as efficient, binder-free, non-noble metal electrocatalysts for the hydrogen evolution reaction (HER) under neutral conditions (pH 7). In particular, the Co9S8/CC material (synthesized in the presence of Zn2+ ions) affords a current density of 10 mA cm−2 at a low overpotential of 175 mV, has great catalytic stability as long as 100 h, and gives about 100% faradaic yield towards the HER in neutral media. The material's excellent catalytic performance toward the HER is attributed primarily to the synergistic effects of Co9S8's intrinsic catalytic ability, the ultrathin nanosheet array architecture and the self-supporting feature.