A novel and general methodology for preparing vertical, complex-oxide nanostructures from a sol–gel-based polymer-precursor solutions is developed using track-etched polymers directly buffering substrates. This method is able to develop a nanostructure over the entire substrate, the dimensions and localization of the vertical nanostructures being preset by the polymeric nanotemplate. Thereby, nanostructures with lateral sizes in the range of 100 to 300 nm and up to 500 nm in height have been grown. Two examples are presented herein, the latter being the evolution of the initial, metastable nanostructure. Specifically, La0.7Sr0.3MnO3 polycrystalline rods are grown at mild temperatures (800 °C); upon strong thermal activation (1000 °C) they suffer a profound transformation into vertical, single-crystalline (La,Sr)xOy nanopyramids sitting on a La0.7Sr0.3MnO3 epitaxial wetting layer. The driving force for this outstanding nanostructural evolution is the minimization of the total energy of the system, which is reached by reducing the grain-boundary, total-surface, and strain-relaxation energies. Finally, advanced electron-microscopy techniques are used to highlight the complex phase separation and structural transformations occurring when the metastable state is overcome.