Developing efficient electrocatalysts for industrial alkaline hydrogen evolution reaction (HER) is important for alleviating energy problems and achieving decarbonization. Here, a multimetal alloyed NiMoCo coating was in situ electrodeposited on Ni mesh as an HER electrocatalyst. The obtained cathode exhibits ultrahigh HER activity in alkaline environments, requiring only low overpotentials of 13.7 and 146.0 mV in 1 M KOH to achieve current densities of 10 and 200 mA cm-2, along with a robust stability of over 120 h at an operating current density of 400 mA cm-2. The introduction of Co elements into NiMo-based alloys could significantly change the surface morphology from a planar to a three-dimensional structure for exposing abundant active sites; moreover, strong electronic interaction between multimetal alloyed sites is beneficial to modulate alkaline HER activity. The full alkaline water splitting electrolyzer, which consisted of electrodeposited multimetal alloyed NiMoCo as the cathode and NiMoFe as the anode, was constructed, showing a cell voltage as low as 1.437 V to obtain the current density of 10 mA cm-2 and 1.703 V for 200 mA cm-2. This work would pave a new avenue to design and fabricate multimetal alloyed electrocatalysts directly grown on an industrial porous conductive substrate.