Oxygen vacancies (V o) in electrocatalysts are closely correlated with the hydrogen evolution reaction (HER) activity. The role of vacancy defects and the effect of their concentration, however, yet remains unclear. Herein, Bi2O3, an unfavorable electrocatalyst for the HER due to a less than ideal hydrogen adsorption Gibbs free energy (ΔG H*), is utilized as a perfect model to explore the function of V o on HER performance. Through a facile plasma irradiation strategy, Bi2O3 nanosheets with different V o concentrations are fabricated to evaluate the influence of defects on the HER process. Unexpectedly, while the generated oxygen vacancies contribute to the enhanced HER performance, higher V o concentrations beyond a saturation value result in a significant drop in HER activity. By tunning the V o concentration in the Bi2O3 nanosheets via adjusting the treatment time, the Bi2O3 catalyst with an optimized oxygen vacancy concentration and detectable charge carrier concentration of 1.52 × 1024 cm−3 demonstrates enhanced HER performance with an overpotential of 174.2 mV to reach 10 mA cm−2, a Tafel slope of 80 mV dec−1, and an exchange current density of 316 mA cm−2 in an alkaline solution, which approaches the top-tier activity among Bi-based HER electrocatalysts. Density-functional theory calculations confirm the preferred adsorption of H* onto Bi2O3 as a function of oxygen chemical potential (∆μ O) and oxygen partial potential (P O2) and reveal that high V o concentrations result in excessive stability of adsorbed hydrogen and hence the inferior HER activity. This study reveals the oxygen vacancy concentration-HER catalytic activity relationship and provides insights into activating catalytically inert materials into highly efficient electrocatalysts.[Figure not available: see fulltext.]