Water-splitting devices for hydrogen generation through electrolysis (hydrogen evolution reaction, HER) hold great promise for clean energy. However, their practical application relies on the development of inexpensive and efficient catalysts to replace precious platinum catalysts. We previously reported that HER can be largely enhanced through finely tuning the energy level of molybdenum sulfide (MoS2) by hot electron injection from plasmonic gold nanoparticles. Under this inspiration, herein, we propose a strategy to improve the HER performance of MoS2 by engineering its energy level via direct transition-metal doping. We find that zinc-doped MoS2 (Zn-MoS2) exhibits superior electrochemical activity toward HER as evidenced by the positively shifted onset potential to -0.13 V vs RHE. A turnover of 15.44 s(-1) at 300 mV overpotential is achieved, which by far exceeds the activity of MoS2 catalysts reported. The large enhancement can be attributed to the synergistic effect of electronic effect (energy level matching) and morphological effect (rich active sites) via thermodynamic and kinetic acceleration, respectively. This design opens up further opportunities for improving electrocatalysts by incorporating promoters, which broadens the understanding toward the optimization of electrocatalytic activity of these unique materials.