The ability to synthesize compositionally complex nanostructures rapidly is akey to high-throughput functional materials discovery. In addition to beingtime-consuming, a majority of conventional materials synthesis processesclosely follow thermodynamics equilibria, which limit the discovery of newclasses of metastable phases such as high entropy oxides (HEO). Herein, aphotonic flash synthesis of HEO nanoparticles at timescales of millisecondsis demonstrated. By leveraging the abrupt heating and cooling cycles inducedby a high-power-density xenon pulsed light, mixed transition metal saltprecursors undergo rapid chemical transformations. Hence, nanoparticlesform within milliseconds with a strong affinity to bind to the carbon substrate.Oxygen evolution reaction (OER) activity measurements of the synthesizednanoparticles demonstrate two orders of magnitude prolonged stability athigh current densities, without noticeable decay in performance, compared tocommercial IrO2catalyst. This superior catalytic activity originates from thesynergistic effect of different alloying elements mixed at a high entropic state.It is found that Cr addition influences surface activity the most by promotinghigher oxidation states, favoring optimal interaction with OER intermediates.The proposed high-throughput method opens new pathways towarddeveloping next-generation functional materials for various electronics,sensing, and environmental applications, in addition to renewable energyconversion.