As a source of clean energy, a reliable hydrogen evolution reaction (HER) requires robust and highly efficient catalysts. Here, by combining chemical vapor transport and Li-intercalation, we have prepared a series of 1T′-phase ReS2xSe2(1-x) (x = 0–1) nanodots to achieve high-performance HER in acid medium. Among them, the 1T′-phase ReSSe nanodot exhibits the highest hydrogen evolution activity, with a Tafel slope of 50.1 mV dec–1 and a low overpotential of 84 mV at current density of 10 mA cm–2. The excellent hydrogen evolution activity is attributed to the optimal hydrogen absorption energy of the active site induced by the asymmetric S vacancy in the highly asymmetric 1T′ crystal structure.