Urchin-like γ-MnO2 nanostructures, composed of nanowires with diameters in the range 40–70 nm were prepared through the direct reaction between MnSO4 and KClO3 via a mild hydrothermal route. Reaction time and temperature were found to influence both the phase and morphology of as-prepared products. For longer reaction times, the initially formed γ-phase transformed to α-MnO2 nanowires along with the loss of urchin-like morphology. Powder x-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy, Fourier transform infrared spectroscopy, and thermogravimetry and differential scanning calorimetry were used to characterize the as-prepared products. On the basis of XRD patterns and SEM images, a possible growth mechanism for the time-dependant morphological evolution of various MnO2 nanostructures has been suggested and discussed.