Highly uniform single-crystal Na-OMS-2 (OMS: octahedral molecular sieve), pyrolusite, and gamma-MnO2 nanostructures with an interesting 3D urchinlike morphology have been successfully prepared using a hydrothermal method based on a mild and direct reaction between sodium dichromate and manganese sulfate. The crystal phases, shapes, and tunnel sizes of the manganese dioxide nanostructures can be tailored. Reaction temperature, concentrations of the reactants, and acidity of the solution play important roles in controlling the synthesis of these manganese dioxides. Field-emission scanning electron microscopy and transmission electron microscopy (TEM) studies show that the nanomaterials obtained are constructed of self-assembled nanorods. X-ray diffraction and TEM results indicate that the constituent manganese dioxide particles are single-crystalline materials. Energy dispersive X-ray analysis and magnetic studies imply that chromium cations may be incorporated into the framework and/or tunnels of the manganese dioxides. A mechanism for the growth of manganese dioxides with urchinlike architectures is proposed.