Display omitted •We proposed a series of transition metal borides (MBenes) monolayer as NRR catalysts.•MBenes exhibit metallic features and electrene characteristics for NRR activity.•Work function of the MBenes can be adopted as a descriptor for the NRR activity. Low-energy consumption and highly selective nitrogen reduction reaction (NRR) catalysts play an important role in solving the limitations of the traditional ammonia production. By means of first-principle calculations, we proposed a series of two-dimensional (2D) transition metal borides (MB) (M = Sc, Ti, V, Y, Zr, Nb, Mo, Hf, Ta and W) monolayer as NRR catalysts. These 2D MBenes exhibit high stability, metallic electronic band structures and the electrene characteristics which contribute to the NRR catalytic activity. Large amounts of active sites accelerate the NRR reaction, and the high selectivity towards NRR inhibits the HER process. We screened out four MBenes: TiB, YB, ZrB and MoB, with favorable limiting overpotentials of 0.64, 0.68, 0.65 and 0.68 V, respectively, which are promising for N2 fixation applications. This work not only enriches the MBene family, but also provides a feasible strategy for the design of NRR catalysts.