AbstractDeveloping low‐cost and efficient oxygen evolution electrocatalysts is key to decarbonization. A facile, surfactant‐free, and gram‐level biomass‐assisted fast heating and cooling synthesis method is reported for synthesizing a series of carbon‐encapsulated dense and uniform FeNi nanoalloys with a single‐phase face‐centered‐cubic solid‐solution crystalline structure and an average particle size of sub‐5 nm. This method also enables precise control of both size and composition. Electrochemical measurements show that among Fe_xNi_(1−x) nanoalloys, Fe_0.5Ni_0.5 has the best performance. Density functional theory calculations support the experimental findings and reveal that the optimally positioned d‐band center of O‐covered Fe_0.5Ni_0.5 renders a half‐filled antibonding state, resulting in moderate binding energies of key reaction intermediates. By increasing the total metal content from 25 to 60 wt%, the 60% Fe_0.5Ni_0.5/40% C shows an extraordinarily low overpotential of 219 mV at 10 mA cm⁻² with a small Tafel slope of 23.2 mV dec⁻¹ for the oxygen evolution reaction, which are much lower than most other FeNi‐based electrocatalysts and even the state‐of‐the‐art RuO₂. It also shows robust durability in an alkaline environment for at least 50 h. The gram‐level fast heating and cooling synthesis method is extendable to a wide range of binary, ternary, quaternary nanoalloys, as well as quinary and denary high‐entropy‐alloy nanoparticles.