Developing highly efficient electrocatalysts and fundamentally understanding the mechanisms for alkaline hydrogen electrode reactions, including the hydrogen oxidation reaction (HOR) and the hydrogen evolution reaction (HER), are highly desirable. Here we report the synthesis of the IrMo alloy nanocatalyst and its outstanding HOR/HER performances under alkaline media. Specifically, the IrMo_0.59 exhibits the highest catalytic activities, which are 5 times higher than that of Ir and commercial Pt/C toward HOR and even a 10-fold enhancement toward HER. Density functional theory (DFT) calculations reveal that H₂O-occupied Mo sites on the surface of IrMo could effectively optimize the free energies of *H₂O and *OH, thereby enable IrMo following the so-called bifunctional mechanism both in alkaline HER/HOR electrolysis.