Single-crystalline nanorods and sea urchin-like morphology of the γ-MnO2 nanostructures were successfully synthesized by hydrothermal method at different synthesis durations. The as-synthesized products were characterized by the techniques X-ray powder diffraction (XRD), field emission gun-scanning electron microscope (FEG-SEM) coupled with energy-dispersive X-ray elemental analysis (EDX), transmission electron microscope (TEM), isotherms of N2 adsorption/desorption and BET-BJH models. The effect of synthesis duration on the morphology, porous structure, and crystallographic form of MnO2 powders was studied. The electrochemical reactivity of as-prepared powders was investigated in 1 mol L−1 KOH by both cyclic voltammetry and impedance spectroscopy by using a micro-cavity electrode. The results show that the best electrochemical reactivity of the MnO2 powder was obtained with synthesis duration of 24 h.