The use of high-valent metal compounds as efficient electrocatalysts for the hydrogen evolution reaction (HER) is rare. Herein, we report that in situ electrochemical activation is an effective strategy to achieve outstanding catalytic activity of TiO₂-coated Co₃O₄ nanowires (TiO₂@Co₃O₄) in the alkaline HER. The activated TiO₂@Co₃O₄ affords a current density of 20 mA cm⁻² at a relatively small overpotential (49 mV), a performance that is superior to that of commercially available Pt/C (20%) and those of most electrocatalysts for the alkaline HER. “Pseudo in situ” spectral characterization and density functional theory calculations suggest that the in situ reduction of Co during the HER results in interstitial Co defects (Co_i) in the TiO₂. In the activated TiO₂@Co₃O₄, water dissociation is efficient at the Co₃O₄/TiO₂ interface, the Co_i defects promote the desorption of the intermediate hydroxyl groups from the TiO₂, and the Tafel step is exothermic on Co₃O₄. Several steps of the HER process are therefore energetically favorable with activated TiO₂@Co₃O₄. The strategy of TiO₂ coating and in situ electrochemical activation is also shown to promote the alkaline HER activity of Co(OH)₂.