Conventional wisdom holds that static disorder is indispensable for the integer quantum Hall effect, underpinning both quantized plateaus and the plateau-plateau transition. We show that pure dephasing, without elastic disorder, is sufficient to generate the same $\theta$-driven criticality. Starting from a Keldysh formulation, we derive an open-system nonlinear $\sigma$ model for class A with a topological $\theta$ term but no cooperon sector, and we demonstrate that nonperturbative instantons still govern a two-parameter flow of $({\sigma }_{xx},{\sigma }_{xy})$. Evaluating $\theta$ in a dephasing quantum anomalous Hall setting, we predict a quantum Hall critical point at ${\sigma }_{xy}=1/2$ with finite ${\sigma }_{xx}$—the hallmark of the integer quantum Hall universality class realized without Anderson localization. Boundary-driven simulations of the Qi-Wu-Zhang model with local dephasing confirm this prediction and provide an experimentally aligned protocol to extract the Hall ratio $\lambda ={\sigma }_{xy}/{\sigma }_{xx}$ from Hall potential maps. By establishing dephasing as a self-contained route to Hall criticality, our framework reframes plateau physics in open solid-state and cold-atom platforms, offering a practical diagnostic for topological transport in nonunitary matter.

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• Nonlinear sigma model
• Open quantum systems & decoherence
• Quantum anomalous Hall effect
• Quantum phase transitions
• Chern insulators