Amorphous materials have garnered significant research interest because of their high structural tolerances and useful functionalities. Here, we develop an effective synthesis method for atomically thin, highly disordered RuO₂ nanosheets that exhibit a promising electrocatalytic performance and a distinct pH-dependent operation mechanism. The poor orbital overlap and coordinatively unsaturated nature of the Ru ions in the highly disordered RuO₂ nanosheets have a synergistic effect on the electrocatalytic performance by enhancing surface adsorption and the activation of lattice oxygen. The highly disordered RuO₂ nanosheets exhibit high electrocatalytic activities in the oxygen evolution reactions (OERs) performed in both alkaline and acidic electrolytes. Various in situ spectroscopic investigations reveal that structural disordering causes a greater contribution of the lattice oxygen participation mechanism in acidic media than in alkaline media. This pH-dependent mechanism can be attributed to the amorphization-induced enhancement of lattice oxygen occupation in the acidic OER medium and increased hydroxide adsorption in the alkaline OER medium. Such disorder-driven pH tuning of the electrocatalytic operation mechanism enables the fabrication of pH-universal high-performance electrocatalysts.