A hydrogenation of N‐heterocycles mediated by diboronic acid with water as the hydrogen atom source is reported. A variety of N‐heterocycles can be hydrogenated with medium to excellent yields within 10 min. Complete deuterium incorporation from stoichiometric D2O onto substrates further exemplifies the H/D atom sources. Mechanism studies reveal that the reduction proceeds with initial 1,2‐addition, in which diboronic acid synergistically activates substrates and water via a six‐membered ring transition state. No metal necessary: An unprecedented expeditious hydrogenation of N‐heterocycles mediated by diboronic acid with water as hydrogen atom source was reported. Mechanism studies revealed that the reduction proceeds with a successive 1,2‐addition, in which diboronic acid synergistically activates substrates and water via a six‐membered ring transition state.