The preparation of Co3O4nanostructures by a green method has been rapidly increasing owing to its promising aspects, such as facileness, atom economy, low cost, scale-up synthesis, environmental friendliness, and minimal use of hazardous chemicals. In this study, we report on the synthesis of Co3O4nanostructures using the milky sap ofCalotropis procera(CP) by a low-temperature aqueous chemical growth method. The milky sap of CP-mediated Co3O4nanostructures were investigated for oxygen evolution reactions (OERs) and supercapacitor applications. The structure and shape characterizations were done by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS) techniques. The prepared Co3O4nanostructures showed a heterogeneous morphology consisting of nanoparticles and large micro clusters. A typical cubic phase and a spinel structure of Co3O4nanostructures were also observed. The OER result was obtained at a low overpotential of 250 mV at 10 mA cm−2and a low Tafel slope of 53 mV dec−1. In addition, the durability of 45 hours was also found at 20 mA cm−2. The newly prepared Co3O4nanostructures using the milky sap of CP were also used to demonstrate a high specific capacitance of 700 F g−1at a current density of 0.8 A g−1and a power density of 30 W h kg−1. The enhanced electrochemical performance of Co3O4nanostructures prepared using the milky sap of CP could be attributed to the surface oxygen vacancies, a relatively high amount of Co2+, the reduction in the optical band gap and the fast charge transfer rate. These surface, structural, and optical properties were induced by reducing, capping, and stabilizing agents from the milky sap of CP. The obtained results of OERs and supercapacitor applications strongly recommend the use of the milky sap of CP for the synthesis of diverse efficient nanostructured materials in a specific application, particularly in energy conversion and storage devices.