Therefore, it is of great importance to develop high-efficiency and stable acidic HER electrocatalysts.Ir exhibits high corrosion resistance and long-term durability under acidic condition, making it a potential candidate electrocatalyst for the HER in acidic electrolytes. [6] However, the poor dispersion and high aggregation of Ir-based materials remain the main obstacles limiting their intrinsic activity. [7] Therefore, it is highly necessary to design the structure and morphology of Ir-based catalysts to achieve superior HER performance. Recently, a new class of ultrathin 2D metallic materials with graphene-like structure, denoted as "metallene," has attracted extensive attention in electrocatalysis. [8] Several metallenes with few-atomic-layer thickness, such as RuRh, [9] PdIr, [10] and RhPdH [11] bimetallene, have been synthesized and showed excellent performance as HER electrocatalysts due to their tunable electronic structures. To further improve the electrocatalytic activity, pore design, interface, and defect engineering are considered effective strategies. [12] Some key properties such as accessible internal surface area, conductivity, diffusion coefficient, and atomic utilization can be improved through pore design and defect engineering, which may significantly improve the performance of electrocatalytic HER. [13] Meanwhile, interfacial engineering of noble metal nanostructures with functional molecules can effectively improve their intrinsic activity. [14] For example, polyallylamine− Rh nanosheet nanoassemblies−carbon nanotubes showed enhanced HER performance because the amino groups of PAH can capture protons in acidic media, leading to proton enrichment in the catalyst/solution interface, which is beneficial for the HER. [15] Inspired by these advances, we propose a one-step wet chemical reduction strategy for the synthesis of polyallylamine-functionalized Ir metallene (Ir@PAH metallene) with ultrathin, layered, highly curved, porous, and defect-rich features. The Ir@ PAH metallene exhibits large electrochemically active surface area, high conductivity, efficient mass transport/diffusion, and tunable electronic structures, resulting in excellent acidic HER performance. The Ir@PAH metallene possesses a low overpotential of only 14 mV at 10 mA cm -2 in 0.5 m H 2 SO 4 solution, which is lower than Pt/C (26 mV), and shows almost no activity decay after cycle stability test.The design of defects and porous structures into metallene with functional surfaces is highly desired to improve its permeability, surface area, and active sites, but remains a great challenge. In this work, polyallylamine-encapsulated Ir metallene with defects and porous structure (Ir@PAH metallene) is easily fabricated by a one-step wet chemical reduction method. The Ir@PAH metallene exhibits excellent hydrogen evolution reaction (HER) performance with an overpotential of only 14 mV at 10 mA cm -2 , a low Tafel slope of 31.2 mV dec -1 , and almost no activity decay after stability test. The abundant defects and pores as well ...