In 1959, Batchelor predicted that passive scalars advected in fluids at finite Reynolds number with small diffusivity κ should display a |k| −1 power spectrum over a small-scale inertial range in a statistically stationary experiment. This prediction has been experimentally and numerically tested extensively in the physics and engineering literature and is a core prediction of passive scalar turbulence.In this article we provide the first mathematically rigorous proof of Batchelor's prediction on the cumulative power spectrum in the κ → 0 limit. We consider fluids governed by the 2D Navier-Stokes equations and 3D hyperviscous Navier-Stokes equations in T d forced by sufficiently regular, nondegenerate stochastic forcing at fixed (arbitrary) Reynolds number. The scalar is subjected to a smooth-inspace, white-in-time stochastic source, and evolves by advection-diffusion with diffusivity κ > 0. Our results rely on the quantitative understanding of Lagrangian chaos and passive scalar mixing established in our recent works. In the κ → 0 limit, we obtain statistically stationary, weak solutions in H − to the stochastically-forced advection problem without diffusivity. These solutions are almost-surely not locally integrable functions, have a non-vanishing average anomalous flux, and satisfy the Batchelor spectrum at all sufficiently small scales. We also prove an Onsager-type criticality result which shows that no such dissipative, weak solutions with a little more regularity can exist.