Pursuing efficient and low-cost electrocatalysts is crucial for the performance of water-alkali electrolyzers toward water splitting. Earth-abundant transition-metal oxides, in spite of their alluring performances in the oxygen evolution reaction, are thought to be inactive in the hydrogen evolution reaction in alkaline media. Here, we demonstrate that pure TiO 2 single crystals, a typical transition-metal oxide, can be activated toward electrocatalytic hydrogen evolution reaction in alkaline media through engineering interfacial oxygen vacancies. Experimental and theoretical results indicate that subsurface oxygen vacancies and low-coordinated Ti ions (Ti 3+ ) can enhance the electrical conductivity and promote electron transfer and hydrogen desorption, which activate reduced TiO 2 single crystals in the hydrogen evolution reaction in alkaline media. This study offers a rational route for developing reduced transition-metal oxides for low-cost and highly active hydrogen evolution reaction catalysts, to realize overall water splitting in alkaline media.