The electrocatalytic nitrogen reduction reaction (NRR) to ammonia (NH3) is a highly desirable yet challenging objective because of the competing hydrogen evolution reaction (HER). Herein, a novel electrocatalyst of Sn‐doped black phosphorene (Sn‐BPene) is demonstrated with dramatically improved selectivity for the NRR. The Sn that is added acts as a sacrificial species for the HER to protect the NRR active sites on black phosphorene (BPene). The Sn‐BPene achieves a Faraday efficiency of up to 36.51% and a prominent NH3 yield rate of 26.98 µg h–1 mgcat–1 at a relatively low overpotential. Density functional theory calculations prove that the adsorption sites of H2O and N2 are separated after doping with Sn, with H2O adsorbing preferentially onto the Sn sites and N2 onto the NRR active sites of BPene, leading to high selectivity. Sn‐doped black phosphorene is developed as an efficient nitrogen reduction reaction (NRR) catalyst that achieves a Faraday efficiency of up to 36.51%, which is an order of magnitude improvement over undoped black phosphorene. Density functional theory calculations prove that the adsorption sites of H2O and N2 are separated after doping with Sn, which provide more opportunities for N2 to adsorb on the NRR‐active sites of black phosphorene, thus leading to high selectivity.