Transition metal selenides are highly promising for clean hydrogen energy generation by overall water electrolysis. Here we report a new approach wherein highly efficient and ultra-stable bifunctional electrocatalyst is developed by synergistic atmospheric-pressure plasma, hydrothermal and selenization treatments of bimetallic electrodes leading to multiphase Ni-Fe-selenide nanosheets (MNFSNs). The remarkable performance in water splitting is evidenced by the low overpotentials for delivering a current density of 10 and 300 mA cm−2 (j10 and j300), which are only 56 and 288 mV for HER, and 200 and 342 mV for OER, respectively, along with robust durability. Moreover, the current densities 10 and 100 mA cm-2 are achieved at low cell voltages of 1.46 and 1.60 V, thus outperforming most of the reported electrocatalysts in two-electrode alkaline water electrolyzers. Ab initio atomistic simulations identify the active catalytic sites formed by Ni atoms located at the heterointerfaces between the FeSe2 and NiSe2 phases.