The development of high-performance Ir-based catalyst for electrocatalysis of oxygen evolution reaction (OER) in acidic media plays a critical role in realizing the commercialization of polymer electrolyte membrane-based water electrolyzer technology. Here we report a low-Ir core–shell OER electrocatalyst consisting of an intermetallic IrGa (IrGa-IMC) core and a partially oxidized Ir (IrO_x) shell. In acidic electrolytes, the IrGa-IMC@IrO_x core–shell catalysts exhibit a low overpotential of 272 mV at 10 mA·cm⁻² with Ir loading of ~20 µg·cm⁻² and a mass activity of 841 A·g_Ir⁻¹ at 1.52 V, which is 3.6 times greater than that of commercial Ir/C (232 A·g_Ir⁻¹) catalyst. We understand by the density functional theory (DFT) calculations that the enhanced OER activity of the IrGa-IMC@IrO_x catalysts is ascribed to the lifted degeneracy of Ir 5d electron of surface IrO_x sites induced by the intermetallic IrGa core, which increases the adsorption capacity of IrO_x layer for O and OH binding and eventually lowers the energy barrier of the OER rate-determining steps.