It is an important issue that exposed active nitrogen atoms (e.g., edge or amino N atoms) in graphitic carbon nitride (g‐C3N4) could participate in ammonia (NH3) synthesis during the photocatalytic nitrogen reduction reaction (NRR). Herein, the experimental results in this work demonstrate that the exposed active N atoms in g‐C3N4 nanosheets can indeed be hydrogenated and contribute to NH3 synthesis during the visible‐light photocatalytic NRR. However, these exposed N atoms can be firmly stabilized through forming BNC coordination by means of B‐doping in g‐C3N4 nanosheets (BCN) with a B‐doping content of 13.8 wt%. Moreover, the formed BNC coordination in g‐C3N4 not only effectively enhances the visible‐light harvesting and suppresses the recombination of photogenerated carriers in g‐C3N4, but also acts as the catalytic active site for N2 adsorption, activation, and hydrogenation. Consequently, the as‐synthesized BCN exhibits high visible‐light‐driven photocatalytic NRR activity, affording an NH3 yield rate of 313.9 µmol g−1 h−1, nearly 10 times of that for pristine g‐C3N4. This work would be helpful for designing and developing high‐efficiency metal‐free NRR catalysts for visible‐light‐driven photocatalytic NH3 synthesis. The BNC coordination formed by boron doping in g‐C3N4 nanosheets is the catalytic active site for the high‐efficiency photocatalytic nitrogen reduction reaction. It also effectively immobilizes the exposed active N atoms of g‐C3N4, inhibits the recombination of photogenerated charges, and improves the visible‐light harvesting efficiency.