Dinitrogen reduction to ammonia using transition metal catalysts is central to both the chemical industry and the Earth's nitrogen cycle. In the Haber–Bosch process, a metallic iron catalyst and high temperatures (400 °C) and pressures (200 atm) are necessary to activate and cleave NN bonds, motivating the search for alternative catalysts that can transform N2 to NH3 under far milder reaction conditions. Here, the successful hydrothermal synthesis of ultrathin TiO2 nanosheets with an abundance of oxygen vacancies and intrinsic compressive strain, achieved through a facile copper‐doping strategy, is reported. These defect‐rich ultrathin anatase nanosheets exhibit remarkable and stable performance for photocatalytic reduction of N2 to NH3 in water, exhibiting photoactivity up to 700 nm. The oxygen vacancies and strain effect allow strong chemisorption and activation of molecular N2 and water, resulting in unusually high rates of NH3 evolution under visible‐light irradiation. Therefore, this study offers a promising and sustainable route for the fixation of atmospheric N2 using solar energy. Ultrathin TiO2 nanosheets with abundant oxygen vacancies (VO) are synthesized through a facile copper‐doping strategy, exhibiting remarkable and stable performance for the photofixation of N2 to NH3 at a rate of 78.9 µmol g−1 h−1 under ambient conditions (especially up to 700 nm). The outstanding activity can be attributed to the existence of VO and compressive strain in the nanosheets.