Ammonia (NH3) is an essential chemical in modern society. It is currently manufactured by the Haber-Bosch process using H-2 and N-2 under extremely high-pressure (>200 bar) and high-temperature (>673 K) conditions. Photo catalytic NH3 production from water and N-2 at atmospheric pressure and room temperature is ideal. Several semiconductor photocatalysts have been proposed, but all suffer from low efficiency. Here we report that a commercially available TiO2 with a large number of surface oxygen vacancies, when photoirradiated by UV light in pure water with N-2, successfully produces NH3. The active sites for N-2 reduction are the Ti3+ species on the oxygen vacancies. These species act as adsorption sites for N-2 and trapping sites for the photoformed conduction band electrons. These properties therefore promote efficient reduction of N-2 to NH3. The solar-to-chemical energy conversion efficiency is 0.02%, which is the highest efficiency among the early reported photocatalytic systems. This noble-metal-free TiO2 system therefore shows a potential as a new artificial photosynthesis for green NH3 production.