Highly efficient, stable and cost-efficient electrocatalysts for hydrogen generation viawater splitting have become in increasing demand for future energy systems. Hitherto, P-rich noble metal polyphosphides which can decrease noble metal (such as Rh, Pd, or Ir) dosage are important to probe potential high-performance HER electrocatalysts. Nevertheless, they are difficult to synthesize at ambient pressure and moderate temperatures. Herein, for the first time, we report a novel iridium diphosphide (IrP₂) electrocatalyst embedded within an ultrathin nitrogen-doped carbon (NC) layer (IrP₂@NC) synthesized at ambient pressure and moderate temperature (900 °C). Subsequent electrochemical tests revealed that such a P-rich IrP₂@NC catalyst possesses the highest hydrogen evolution reaction (HER) activity among all the documented transition metal phosphide electrocatalysts, including the commercial Pt/C, with ultralow overpotentials of 8 and 28 mV to achieve 10 mA cm^?2 in 0.5 M H₂SO₄ and 1.0 M KOH, respectively. Combined density functional theory (DFT) computational studies suggest that the introduction of phosphorus into iridium can weaken the H adsorption strength of IrP₂, beneficial for boosting HER activity. More importantly, this synthetic strategy for P-rich IrP₂@NC can also be applied to other noble metal diphosphides (RhP₂@NC and Pd₅P₂@NC, etc.). This work presents a particularly efficient and stable P-rich transition metal polyphosphide with advanced HER performance and beyond.