Among many types of low-dimensional inorganic nanostructures, nanostructured MnO₂ provides unique advantages in designing and synthesizing efficient electrode and catalyst materials for novel energy storage technologies. The low cost, high redox potential, theoretically high energy storage capacity, environmentally friendliness, and rich deposits provide MnO₂ nanomaterials with a high level of economic feasibility for various renewable energy-related applications, like electrodes for metal-ion batteries/metal–sulfur batteries/metal–O₂ batteries/supercapacitors and electrocatalysts for oxygen evolution/oxygen reduction/CO₂ reduction/N₂ fixation. In this review, a wide spectrum of the energy-related applications of diverse nanostructured MnO₂ materials is systematically surveyed along with versatile synthetic methods of these metal oxides and chemical design strategies to improve their electrochemical functionalities. Future research perspectives for nanostructured MnO₂ materials are provided to offer insightful directions for the exploration of next-generation energy storage/conversion systems.