||Transition metal dichalcogenides (TMDs) have been investigated for use in a hydrogen evolution reaction (HER), mostly in the form of nano-sized flakes, due to the abundant active site formation by nanostructuring, surface functionalization, and phase engineering. However, the physical origin of the active HER on TMDs remains to be clarified. Here, we investigate the role of anion vacancies for the HER on the basal plane of single-crystalline tungsten dichalcogenide (WTe2), a group 6 metallic TMD. The WTe2 with a small amount of anionic (Te) vacancies shows an improved overpotential from -0.707 to -0.568 V and a constant Tafel slope of 154 mV/dec in the HER. Photoemission spectroscopy, combined with first-principle calculations, reveals that the work function of WTe2 is decreased by the anionic Te vacancies, which improves the bulk conductivity and the overpotential in the HER with the material. Moreover, the enlarged electrochemical active surface area with a large number of Te vacancies in the WTe2 critically improves the HER performance with decreases in the overpotential and the Tafel slope, -0.119 V and 79 mV/dec, respectively. Our results show that the modulation of work function and surface morphology is a promising way to improve the HER in TMDs.