A universal, simple, robust, widely applicable and cost‐effective aqueous process is described for a controlled oxidative dissolution process of micrometer‐sized metal powders to form high‐purity aqueous dispersions of colloidally stable 3–8 nm metal oxide nanoparticles. Their utilization for making single and multilayer optically transparent high‐surface‐area nanoporous films is demonstrated. This facile synthesis is anticipated to find numerous applications in materials science, engineering, and nanomedicine.