Renewable energy-driven electrochemical N-2 reduction reaction (NRR) provides a green and sustainable route for NH3 synthesis under ambient conditions but is plagued by a high reaction barrier and low selectivity. To promote NRR, modification of the catalyst surface to increase N-2 adsorption and activation is key. Here, we show that engineering surface oxygen vacancies of TiO2 permits significantly enhanced NRR activity with an NH3 yield rate of about 3.0 mu g(NH3)h(-)(1 )mg(cat.)(-1) and a faradaic efficiency (FE) of 6.5% at -0.12 V (vs. the reversible hydrogen electrode, RHE). Efficient conversion of N-2 to NH3 is achieved in a wide applied potential range from -0.07 to -0.22 V (vs. RHE) with NH3 production rates >= 2.0 RgNH(3 )mu g(NH3)h(-)(1 )mg(cat.)(-1) and NH3 FEs >= 4.9%, respec- tively. An NH3 FE as high as 9.8% is obtained at a low overpotential of 80 mV. Density functional theory calculations reveal that the surface oxygen vacancies in TiO2 play a vital role in facilitating electrochemical N-2 reduction by activating the first protonation step and also increasing N-2 chemisorption (relative to *H).